Criticising volunteers about changing course (9)
I believe the answer is:
attacking
'criticising' is the definition.
(attacking is a kind of criticising)
'volunteers about changing course' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'about' shows that the letters should be reversed in order.
'changing course' becomes 'tacking' (I've seen this before).
'ta' in reverse letter order is 'at'.
'at'+'tacking'='ATTACKING'
(Other definitions for attacking that I've seen before include "Assailing" , "offensive" , "Assaulting" .)