Assumes spat developed outside party (6)
I believe the answer is:
adopts
'assumes' is the definition.
('adopt' can be a synonym of 'assume')
'spat developed outside party' is the wordplay.
'developed' indicates anagramming the letters.
'outside' indicates putting letters inside.
'party' becomes 'do' (do is a kind of party).
'spat' anagrammed gives 'apts'.
'apts' placed around 'do' is 'ADOPTS'.
(Other definitions for adopts that I've seen before include "Selects" , "Takes a child into one's family" , "Legally takes on (child)" , "Approves or accepts, a motion or accounts" , "Takes into one's family legally" .)