Without notice, alter fitting (4)
I believe the answer is:
just
'fitting' is the definition.
(similar in meaning)
'without notice alter' is the wordplay.
'without' suggests deleting specific letters.
'notice' becomes 'ad' (both can mean an advertisement).
'alter' becomes 'adjust' (both can mean to modify).
'adjust' with 'ad' removed is 'JUST'.
(Other definitions for just that I've seen before include "Simply - fair" , "Merely right?" , "And nothing more" , "Quite" , "Fair - precisely" .)