Clue: Union man's bedroom frolics with girl left out (10)
Answer: BRIDEGROOM
Explanation: Definition: Union man
***(bedroom gir)** where the GIR is "girl" with L ("left") *out*. 
*I want to like this clue as it has a clever surface and a witty definition but the ROOM appearing in the same order in GROOM and BEDROOM makes the anagram a bit too obvious.