complementary function to unapply()

[leprechaun]

I need to 'extract' a function's underlying expression. If you want the complementary function to unapply(). When an Expression is given I can convert it into a function with unapply(expr, var1, var2 ,...). I would I do that vice versa?
I did try to extract the Expression from the function Definition, but in order to use mid() or other string related functions I need a string first. If I have fun(x,y):=x*y it is represented as (x,y)->x*y.
STRING(fun(x,y)) works and evaluates into "x*y" but entering the variables is cumbersome and error-prone so I would very much prefer STRING(fun). How do you do it? Or is there a built in function related to unapply wich does the Job?