Wind-up? Why to be wary (5)
I believe the answer is:
leery
'be wary' is the definition.
(leery can mean wary or cautious)
'wind-up? why' is the wordplay.
'wind' becomes 'reel' (to reel something is to wind it up).
'up?' shows that the letters should be reversed in order (in down clue: letters go upwards).
'why' becomes 'y'.
'reel' backwards is 'leer'.
'leer'+'y'='LEERY'
'to' acts as a link.
(Other definitions for leery that I've seen before include "Cautious due to suspicions" , "Knowing" , "Cunning - wary" , "Sly, cunning" , "Shy" .)