Heavy defeat — manager finally dismissed (4)
I believe the answer is:
rout
'heavy defeat' is the definition.
(I've seen this before)
'manager finally dismissed' is the wordplay.
'finally' indicates one should take the final letters.
'dismissed' becomes 'out' (dismissed in cricket).
The final letter of 'manager' is 'r'.
'r'+'out'='ROUT'
(Other definitions for rout that I've seen before include "Whitewash" , "Defeat comprehensively" , "Crushing defeat" , "Big defeat" , "Decisive defeat" .)