Prick! Sackings extremely gratuitous (8)
I believe the answer is:
'gratuitous' is the definition.
(gratuitous can mean needless or uncalled for)
'prick sackings extremely' is the wordplay.
'prick' becomes 'needle' (needling is a kind of pricking).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'sackings' with its middle removed is 'ss'.
(Other definitions for needless that I've seen before include "Gratuitous" , "Surplus to requirements" , "- - to say" , "without provocation?" , "uncalled for" .)