Part of solenoid leaking is not working (4)
I believe the answer is:
idle
'is not working' is the definition.
(I've seen this before)
'part of solenoid leaking' is the wordplay.
'part of' says the answer is hidden in the clue.
'IDLE' is hidden within 'solenoid leaking'.
(Other definitions for idle that I've seen before include "having been taken from service" , "Maybe Eric" , "not being engaged" , "Lazy - vain" , "Tick over like engine" .)