Detest bowler, say, opening for Essex (4)
I believe the answer is:
hate
'detest' is the definition.
('hate' can be a synonym of 'detest')
'bowler say opening for essex' is the wordplay.
'bowler say' becomes 'hat' (bowler is a kind of hat**).
'opening for' suggests taking the first letters.
The first letter of 'essex' is 'e'.
'hat'+'e'='HATE'
(Other definitions for hate that I've seen before include "Antipathy" , "Despise" , "Intense dislike" , "Strong aversion" , "Abhor" .)