Change answer drained teacher's put before class (9)
I believe the answer is:
transform
'change' is the definition.
(transforming is a kind of changing)
'answer drained teacher's put before class' is the wordplay.
'answer' becomes 'ans' (A common abbreviation).
'drained' means to remove the middle letters.
'put before' means one lot of letters go next to another.
'class' becomes 'form' (synonyms).
'teacher' with its middle taken out is 'tr'.
'ans' put after 'tr' is 'trans'.
'trans'+'form'='TRANSFORM'
(Other definitions for transform that I've seen before include "Change completely" , "Change the voltage I" , "Make great change in appearance of" , "Subject to change" , "Change markedly" .)