Sharp matron bustling round ward, finally (7)
I believe the answer is:
mordant
'sharp' is the definition.
(thesaurus)
'matron bustling round ward finally' is the wordplay.
'bustling' indicates anagramming the letters (letters bustle around).
'round' indicates putting letters inside.
'finally' indicates one should take the final letters.
The last letter of 'ward' is 'd'.
'matron' anagrammed gives 'morant'.
'morant' enclosing 'd' is 'MORDANT'.
(Other definitions for mordant that I've seen before include "Caustic" , "Biting, scathing" , "Acerbic" , "Waspish" , "Bitingly sarcastic" .)