Study once more the advert (4)
I believe the answer is:
read
'study' is the definition.
(synonyms)
'once more the advert' is the wordplay.
'once more' becomes 're' (prefix in words like 're-emerge' or 're-enter').
'the advert' becomes 'ad' (synonyms).
're'+'ad'='READ'
(Other definitions for read that I've seen before include "Understand written music" , "what follows 18 [OMASUM] in a 17 [RUMINANT]" , "Speak" , "Measure energy or fuel consumption" , "Perused" .)