As Garbo did, longed to be different at heart (5)
I believe the answer is:
acted
'as garbo did' is the definition.
(Greta Garbo was an actress)
I don't understand how the remainder of the clue works.
(Other definitions for acted that I've seen before include "participated in panto maybe" , "Performed on stage" , "Performed (in a play)" , "Played a role" , "produced an effect" .)