Criticise reform inside of borstal (5)
I believe the answer is:
roast
'criticise' is the definition.
(thesaurus)
'reform inside of borstal' is the wordplay.
'reform' is an anagram indicator.
'inside of' says to take the centre.
The middle of 'borstal' is 'orsta'.
'orsta' with letters rearranged gives 'ROAST'.
(Other definitions for roast that I've seen before include "Become very warm" , "Reprimand severely" , "Cook in oven or by exposure to heat" , "Oven joint" , "Oven-cook, tease" .)