Former pupil's usual composition about manganese (7)
I believe the answer is:
alumnus
'former pupil's' is the definition.
(I've seen this before)
'usual composition about manganese' is the wordplay.
'composition' indicates an anagram.
'about' means one lot of letters goes inside another.
'manganese' becomes 'Mn' (Mn is the chemical symbol for manganese).
'usual' is an anagram of 'aluus'.
'aluus' enclosing 'mn' is 'ALUMNUS'.
(Other definitions for alumnus that I've seen before include "Ex-pupil" , "(US) college graduate" , "Former pupil or student" , "Past pupil" , "Former school pupil" .)