Bats briefly at Lord’s, overly trusting partner? (7)
I believe the answer is:
cuckold
'overly trusting partner?' is the definition.
(someone whose partner commits adultery)
'bats briefly at lord's' is the wordplay.
'bats' becomes 'cuckoo' (both can mean mad).
'briefly' means to remove the last letter.
'at' means one lot of letters go next to another.
'lord' becomes 'ld' (abbreviation for the aristocratic title).
'cuckoo' with its final letter taken off is 'cucko'.
'cucko'+'ld'='CUCKOLD'
(Other definitions for cuckold that I've seen before include "Spouse of an adulterous wife" , "One cheated" , "Man whos wife is cheating on him?" , "fool of a husband" , "cheat on hubbies" .)