Gin cocktail is a constituent of happy party (10)
I believe the answer is:
contingent
'party' is the definition.
(I've seen this before)
'gin cocktail is a constituent of happy' is the wordplay.
I cannot really see how this works, but
an anagram of 'gin' is 'ing' which is present in the answer.
'of' could be 'o' and 'o' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for contingent that I've seen before include "Some soldiers" , "party" , "Subject to chance" , "not necessary" , "Company" .)