Old king cheers queen undoing trousers (7)
I believe the answer is:
tarquin
'old king' is the definition.
(legendary king of Rome)
'cheers queen undoing trousers' is the wordplay.
'cheers' becomes 'ta' (both can informally mean thank you).
'queen' becomes 'q' (abbreviation used in chess).
'undoing' becomes 'ruin' (both can mean someone's downfall).
'trousers' means one lot of letters goes inside another (to trouser can mean to contain or hold).
'q' placed within 'ruin' is 'rquin'.
'ta'+'rquin'='TARQUIN'
(Other definitions for tarquin that I've seen before include "proud king" , "Last Etruscan king of Rome, 534-510 BC" .)