Through in deviation (3)
I believe the answer is:
via
'through' is the definition.
(via means going through)
'in deviation' is the wordplay.
'in' indicates a hidden word.
'VIA' is hidden within 'deviation'.
(Other definitions for via that I've seen before include "By means of" , "Passing through" , "By way of - in Latin" , "Going through" , "A Roman way" .)