Thick bagel given to a school member (6)
I believe the answer is:
oafish
'thick' is the definition.
(thesaurus)
'bagel given to a school member' is the wordplay.
'bagel given' becomes 'oa' (I am not sure about this - if you are sure you should believe this answer much more).
'to a school member' becomes 'fish' (school can mean a group of fish).
'oa'+'fish'='OAFISH'
(Other definitions for oafish that I've seen before include "Like a lout" , "Like a boorish person" , "Bone-headed" , "lacking elegance?" , "crass" .)