Waspish matron fussing round head of department (7)
I believe the answer is:
mordant
'waspish' is the definition.
(both can mean sardonic or acerbic)
'matron fussing round head of department' is the wordplay.
'fussing' indicates an anagram (fuss can mean to disturb).
'round' is an insertion indicator.
'head of' indicates taking the first letters.
The initial letter of 'department' is 'd'.
'matron' with letters rearranged gives 'morant'.
'morant' enclosing 'd' is 'MORDANT'.
(Other definitions for mordant that I've seen before include "Acrimonious" , "Bitingly sarcastic" , "Biting, scathing" , "Waspish" , "(Of wit) sharp, critical" .)