Suitable old stamp's found in exercise books (9)
I believe the answer is:
expedient
'suitable' is the definition.
(I've seen this before)
'old stamp's found in exercise books' is the wordplay.
'old' becomes 'ex'.
'stamp' becomes 'die' (I've seen this in other clues).
'found in' is an insertion indicator.
'exercise' becomes 'pe' (Physical Education).
'books' becomes 'nt' (New Testament).
'pe'+'nt'='pent'
'die' going into 'pent' is 'pedient'.
'ex'+'pedient'='EXPEDIENT'
(Other definitions for expedient that I've seen before include "Opportune" , "Convenient, practical" , "shift" , "Convenient for a particular purpose" , "Handy" .)