Trap a king — what's stopping merry old one? (8)
I believe the answer is:
cakehole
'trap' is the definition.
(both can informally mean someone's mouth)
'a king what's stopping merry old one?' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'stopping' is an insertion indicator (inserted letters stop or plug a gap).
'merry old one?' becomes 'cole' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'k'+'eh'='akeh'
'akeh' placed into 'cole' is 'CAKEHOLE'.
Can you help me to learn more?
(Other definitions for cakehole that I've seen before include "Chops" , "Echo leak (anag) - gob" , "Gob (slang)" , "Mouth (slang)" , "What a doughnut might have" .)