Spur for one mostly departed (3,2)
I believe the answer is:
egg on
'spur' is the definition.
(I've seen this in another clue)
'for one mostly departed' is the wordplay.
'for one' becomes 'eg' (both can mean 'for example').
'mostly' means to remove the last letter (most but not all of the word).
'departed' becomes 'gone' ('go' can be a synonym of 'depart').
'gone' with its final letter removed is 'gon'.
'eg'+'gon'='EGG ON'
(Other definitions for egg on that I've seen before include "One cooking breakfast might have this" , "Urge, impel" , "Fire" , "Encourage to act" , "this face is obvious embarrassment" .)