Settle bill exactly (6)
I believe the answer is:
adjust
'settle' is the definition.
(thesaurus)
'bill exactly' is the wordplay.
'bill' becomes 'ad' (advertisement stuck on wall).
'exactly' becomes 'just' (synonyms).
'ad'+'just'='ADJUST'
(Other definitions for adjust that I've seen before include "Adapt (to new situation)" , "Calibrate or regulate" , "Regulate, alter by small amount" , "Adapt, regulate" , "Tune" .)