Total bill before course gets sent back (3,2)
I believe the answer is:
add up
'total' is the definition.
(synonyms)
'bill before course gets sent back' is the wordplay.
'bill' becomes 'ad' (advertisement stuck on wall).
'before' is a charade indicator (letters next to each other).
'course' becomes 'pud' (pud is a kind of course).
'gets sent back' says the letters should be written backwards.
'pud' backwards is 'dup'.
'ad'+'dup'='ADD UP'
(Other definitions for add up that I've seen before include "Ring true" , "Do sums" , "tot" , "Seem to be sensible" , "Sum - make sense" .)