Commonest part of appropriate assistance was worth doing (4)
I believe the answer is:
paid
'commonest part' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I don't see?
'appropriate assistance was worth doing' is the wordplay.
I cannot really understand how this works, but
'assistance' could be 'aid' (synonyms) and 'aid' is present in the answer.
A single letter 'p' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for paid that I've seen before include "Remunerated" , "Settled debt" , "Hired" , "Discharged a debt" , "See 12" .)