Become dormant (9)
I believe the answer is:
hibernate
'become dormant' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hibernate that I've seen before include "go out in the snow?" , "Become dormant" , "Sleep durning winter" , "Pass the winter in a dormant state" , "Breathe in for long winter sleep" .)