Study chapter on a gastropod (5)
I believe the answer is:
conch
'a gastropod' is the definition.
(conch is a kind of gastropod)
'study chapter on' is the wordplay.
'study' becomes 'con' (conning is a kind of studying).
'chapter' becomes 'ch'.
'on' means one lot of letters go next to another.
'con'+'ch'='CONCH'
(Other definitions for conch that I've seen before include "Type of shell" , "Marine mollusc with a spiral shell" , "Large spiral shell" , "Large shell used as trumpet" , "One's sometimes pulled in to beach" .)