Assume exercise will be needed after a party (5)
I believe the answer is:
adopt
'assume' is the definition.
(I know that assume can be written as adopt)
'exercise will be needed after a party' is the wordplay.
'exercise' becomes 'PT' (Physical Training).
'will be needed after' says to put letters next to each other.
'party' becomes 'do' (do is a kind of party).
'a'+'do'='ado'
'pt' put after 'ado' is 'ADOPT'.
(Other definitions for adopt that I've seen before include "Foster" , "Take on (another's child)" , "Choose as one's own" , "Accept as one's own" , "Take responsibility for" .)