How do married clergy (preachers) in Protestant denominations support their family? In Roman Catholicism the clergy is predominantly celibate and hence there is no pressure of sustaining a family.  

What exactly is the system in a Protestant clergy? Are they employed in a gainful job, in addition to being involved in preaching? If so, how do they make time for God as well as for family? How do they sustain their family?
There's no singular answer to this question, and you'd likely see tons of different situations depending on the pastor and his church. 

At my church, the pastor is full-time, and his salary is paid out of a portion of the tithes given to the church.  However, in some situations, pastors must find another job in order to pay the bills, typically because they're working in a smaller church where the tithes are not large enough to support him.  I've known pastors who are in this situation as well.  They'll work a factory job or something low-key and low-demand so that they can support themselves as they typically treat their clergy role as their "primary" job, even though it doesn't pay well enough to support a family.

Some protestant churches, however, are also set up in denominations, where the salary is paid by the denomination, and tithe money typically rolls into the denomination and not directly into the church.   It just depends on the situation. 

Each church is different. 

As for balancing time, again, just like any job, some have effective time management, and some don't.  My brother is a pastor, and he manages his time with his family by simply carving out time that is devoted to the family, and regularly blocking that time out so that he can properly perform his role as father and husband.  Other pastors may not have the same time management skills, and may indeed neglect their families to serve a "higher call".  This is an unfortunate fact that happens all too often (hence the stereotype of the "Pastor's Kid", who ends up being a wild child, largely due to his father's neglect).  

Ultimately, the married clergyman would have the same approach to supporting his family as the unmarried clergyman would.  He would have to trust God to meet his needs, as well as the needs of his family if he has one.