School’s study holding a hundred back (7)
I believe the answer is:
educate
'school's' is the definition.
(synonyms)
'study holding a hundred back' is the wordplay.
'study' becomes 'etude' (I've seen this before).
'holding' indicates putting letters inside.
'hundred' becomes 'c' (Roman numeral).
'back' says the letters should be written in reverse.
'a'+'c'='ac'
'etude' going around 'ac' is 'etacude'.
'etacude' backwards is 'EDUCATE'.
(Other definitions for educate that I've seen before include "Enlighten" , "Train, instruct" , "Train, teach" , "Give instruction to" , "Team" .)