Appropriate when likely to hold a party (5)
I believe the answer is:
adopt
'appropriate' is the definition.
(thesaurus)
'likely to hold a party' is the wordplay.
'likely' becomes 'apt' (similar in meaning).
'to hold' is an insertion indicator.
'a party' becomes 'do' (do is a kind of party).
'apt' placed around 'do' is 'ADOPT'.
'when' is the link.
(Other definitions for adopt that I've seen before include "Embrace" , "Take on (another's child)" , "Choose as one's own" , "Accept as one's own" , "Espouse" .)