Labour stronghold supports left wingers quitting (6)
I believe the answer is:
effort
'labour' is the definition.
(effort is a kind of labour)
'stronghold supports left wingers quitting' is the wordplay.
I cannot really see how this works, but
'stronghold' could be 'fort' (I've seen this before) and 'fort' is located in the answer.
The remaining letters 'ef' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for effort that I've seen before include "Exertion, elbow grease" , "Earnest attempt" , "Vigorous attempt" , "Toil" , "Work - application" .)