Suitable exercises cut out in old books (9)
I believe the answer is:
expedient
'suitable' is the definition.
(I've seen this before)
'exercises cut out in old books' is the wordplay.
'exercises' becomes 'pe' (Physical Education class).
'cut out' becomes 'die' (dying is a kind of cutting out).
'in' is an insertion indicator.
'old' becomes 'ex' (prefix meaning former or onetime).
'books' becomes 'NT' (New Testament).
'pe'+'die'='pedie'
'ex'+'nt'='exnt'
'pedie' going within 'exnt' is 'EXPEDIENT'.
(Other definitions for expedient that I've seen before include "Means to get" , "shift" , "Suitable for the circumstances rather than just" , "Suited to the circumstances" , "Convenient for a particular purpose" .)