Minister assuming party position (3-2)
I believe the answer is:
mid-on
'position' is the definition.
(mid-on is a kind of position)
'minister assuming party' is the wordplay.
'minister' becomes 'min' (I've seen this in other clues).
'assuming' is an insertion indicator (inserted letters are taken in or assumed).
'party' becomes 'do' (do is a kind of party).
'min' placed around 'do' is 'MID-ON'.
(Other definitions for mid-on that I've seen before include "Fielding position" , "Leg side fieldsman" , "Cricket fielder" , "Cricketing position" .)