Bubbly guy gutted to admit halfwit (5)
I believe the answer is:
gassy
'bubbly' is the definition.
'guy gutted to admit halfwit' is the wordplay.
'gutted' suggests removing the centre (to gut something is to remove its insides).
'to admit' is an insertion indicator.
'halfwit' becomes 'ass' (I've seen this in another clue).
'guy' with its centre taken out is 'gy'.
'gy' enclosing 'ass' is 'GASSY'.
(Other definitions for gassy that I've seen before include "Carbonated" , "Fizzy" , "Verbose" .)