Sign I'm surprised crossing northerly tracks (7)
I believe the answer is:
gesture
'sign' is the definition.
(I know that sign is a more specific form of the action gesture)
'i'm surprised crossing northerly tracks' is the wordplay.
'i'm surprised' becomes 'gee' (exclamation of surprise).
'crossing' is an insertion indicator (to cross can mean to go over or around).
'northerly' says the letters should be written backwards (in a down clue, letters go up or 'north').
'tracks' becomes 'ruts' (rut can mean a track).
'ruts' back-to-front is 'stur'.
'gee' enclosing 'stur' is 'GESTURE'.
(Other definitions for gesture that I've seen before include "Gesticulate, sign" , "Hand movement to communicate" , "Hand motion" , "Movement to express meaning" , "wave, perhaps?" .)