Change exercises after missing one class (9)
I believe the answer is:
transform
'change' is the definition.
(I know that transform is a more specific form of the action change)
'exercises after missing one class' is the wordplay.
'exercises' becomes 'trains' (train can mean to do exercise).
'after missing' suggests deleting specific letters.
'one' becomes 'I' (Roman numeral).
'class' becomes 'form class' (form class is a kind of class).
'trains' with 'i' taken away is 'trans'.
'trans'+'form'='TRANSFORM'
(Other definitions for transform that I've seen before include "Change the voltage I" , "Subject to change" , "Change markedly" , "Change completely" , "Make great change in appearance of" .)