Long hours in service that's unstoppable (4)
I believe the answer is:
ache
'long' is the definition.
(synonyms)
'hours in service that's unstoppable' is the wordplay.
I cannot quite see how this works, but
'hours' could be 'h' and 'h' is found within the answer.
'service' could be 'ace' (ace is a kind of service) and 'ace' is found in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ache that I've seen before include "Play up" , "Throb - hurt" , "Longing" , "Cause pain" , "Pine" .)