Possible boiler trouble (3,5)
I believe the answer is:
hot water
This clue is a double definition.
'possible boiler' is the first definition.
(I've seen this before)
'trouble' is the second definition.
(to get in hot water is to get in trouble)
(Other definitions for hot water that I've seen before include "(Liquid) trouble!" , "Uncomfortable predicament to be in?" , "Difficulty" , "Predicament to be avoided (informal)" , "Taps contents" .)