Fellow to criticise one set of drums (7)
I believe the answer is:
timpani
'set of drums' is the definition.
'fellow to criticise one' is the wordplay.
'fellow' becomes 'tim' (I have seen 'Little fellow' mean 'tim' so perhaps 'fellow' could also mean 'tim').
'to criticise' becomes 'pan' (informal term meaning to criticise heavily).
'one' becomes 'i' (Roman numeral).
'tim'+'pan'+'i'='TIMPANI'
(Other definitions for timpani that I've seen before include "Features of kitchen" , "orchestra members" , "Set of kettledrums" , "Percussion section" , "PM, I ain't (anag.)" .)