Blast fellow switching sides during contest (6)
I believe the answer is:
berate
'blast' is the definition.
(both can mean to criticise)
'fellow switching sides during contest' is the wordplay.
'fellow switching sides' becomes 'rat' (rat can mean a traitor).
'during' means one lot of letters goes inside another.
'contest' becomes 'bee' (I've seen this in another clue).
'rat' put into 'bee' is 'BERATE'.
(Other definitions for berate that I've seen before include "Rebuke" , "Have words with" , "Scold about rebate" , "Condemn" , "Censure severely or angrily" .)