Studied once more the advert (4)
I believe the answer is:
read
'studied' is the definition.
(to read a subject is to study it at university)
'once more the advert' is the wordplay.
'once more' becomes 're' (prefix in words like 're-emerge' or 're-enter').
'the advert' becomes 'ad' ('ad' can be a synonym of 'advert').
're'+'ad'='READ'
(Other definitions for read that I've seen before include "Measure energy or fuel consumption" , "Lecture" , "examined" , "register" , "Study (subject at university)" .)