Prick! Sackings extremely gratuitous (8)
I believe the answer is:
needless
'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'.
'needle'+'ss'='NEEDLESS'
(Other definitions for needless that I've seen before include "Gratuitous" , "Surplus to requirements" , "- - to say" , "without provocation?" , "uncalled for" .)