Crackling relating to small hole in humbug (9)
I believe the answer is:
crepitant
'crackling' is the definition.
(I've seen this in another clue)
'relating to small hole in humbug' is the wordplay.
'relating to small' becomes 're' (re can mean with regards to; 'small' suggests an abbreviation).
'hole' becomes 'pit' (pit is a kind of hole).
'in' indicates putting letters inside.
'humbug' becomes 'can't' (I've seen this in another clue).
're'+'pit'='repit'
'repit' inserted inside 'cant' is 'CREPITANT'.
(Another definition for crepitant that I've seen is " crackling!".)