Large doubt mostly that Bill puts on hairspray (7)
I believe the answer is:
lacquer
'hairspray' is the definition.
(I've seen this before)
'large doubt mostly that bill puts on' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'doubt' becomes 'query' (a query is a doubt or question).
'mostly' means to remove the last letter (most but not all of the word).
'that bill' becomes 'ac' (abbreviation for account).
'puts on' indicates putting letters inside (some letters put on or wear other letters).
'l'+'query'='lquery'
'lquery' with its last letter removed is 'lquer'.
'lquer' placed around 'ac' is 'LACQUER'.
(Other definitions for lacquer that I've seen before include "Shellac and alcohol hair spray" , "Japan, for example" , "Hard coating for wood" , "protective cover" , "Hair treatment; tree sap" .)