Mr Benn's overwhelming sex appeal leading to merriment (8)
I believe the answer is:
hilarity
'merriment' is the definition.
(I know that hilarity is a type of merriment)
'mr benn's overwhelming sex appeal' is the wordplay.
'mr benn' becomes 'Hilary' (politician Hilary Benn).
'overwhelming' means one lot of letters goes inside another.
'sex appeal' becomes 'it'.
'hilary' going around 'it' is 'HILARITY'.
'leading to' is the link.
(Other definitions for hilarity that I've seen before include "Gaiety" , "Absence of gravity" , "Great amusement" , "Laughter" , "fun" .)