Take up, when likely to be out of the party (5)
I believe the answer is:
adopt
'take up' is the definition.
('adopt' can be a synonym of 'take up')
'likely to be out of the party' is the wordplay.
'likely' becomes 'apt' ('apt' can be similar in meaning to 'likely').
'to be out of' means one lot of letters goes inside another.
'the party' becomes 'do' (do is a kind of party).
'apt' going around 'do' is 'ADOPT'.
'when' is the link.
(Other definitions for adopt that I've seen before include "make one's own" , "Choose as one's own" , "Take responsibility for" , "Accept as one's own" , "Assume (attitude)" .)