Gym expected to involve eager teacher (9)
I believe the answer is:
pedagogue
'teacher' is the definition.
(I've seen this before)
'gym expected to involve eager' is the wordplay.
'gym' becomes 'pe' (short for Physical Education).
'expected' becomes 'due' (similar in meaning).
'to involve' means one lot of letters goes inside another (inserted letters get involved).
'eager' becomes 'agog' (I've seen this before).
'due' going around 'agog' is 'dagogue'.
'pe'+'dagogue'='PEDAGOGUE'
(Other definitions for pedagogue that I've seen before include "Strict teacher" , "Educator of young people" , "Dogmatic teacher" , "Schoolmaster" .)