Ship’s propeller with power supply trouble (8)
I believe the answer is:
mainsail
'ship's propeller' is the definition.
'with power supply trouble' is the wordplay.
'with' is a charade indicator (letters next to each other).
'power supply' becomes 'mains' (I've seen this before).
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble').
'mains' put next to 'ail' is 'MAINSAIL'.
(Other definitions for mainsail that I've seen before include "Ship's largest canvas" , "The principal sheet on board" , "key equipment on board" , "Principal ship's canvas" , "Central piece of rigging" .)