Fiercely criticize barman in store (6)
I believe the answer is:
savage
'fiercely criticize' is the definition.
'savage' can be an answer for 'criticize' (savaging is a kind of criticizing). I'm not certain of the 'fiercely' bit.
'barman in store' is the wordplay.
'barman' becomes 'ag' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'store' becomes 'save' (I've seen this before).
'ag' put into 'save' is 'SAVAGE'.
Can you help me to learn more?
(Other definitions for savage that I've seen before include "Uncivilised, vicious" , "Viciously criticise" , "poet who escaped death penalty" , "Cruel, vicious" , "Uncivilised and fierce" .)