A topping fellow at school (4)
I believe the answer is:
head
'a topping' is the definition.
(thesaurus)
'fellow at school' is the wordplay.
'fellow' becomes 'he' ('he' refers to a man).
'at school' becomes 'ad' (I can't explain this - if you can you should give a lot more credence to this answer).
'he'+'ad'='HEAD'
Can you help me to learn more?
(Other definitions for head that I've seen before include "River or stream source" , "Person in charge" , "''and still the wonder grew / That one small ... could carry all he knew'' (Goldsmith)" , "School boss" , "Chief; promontory" .)