Breathe in, out, sleep long and deep (9)
I believe the answer is:
hibernate
'sleep long and deep' is the definition.
'breathe in out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'breathe'+'in'='breathein'
'breathein' with letters rearranged gives 'HIBERNATE'.
(Other definitions for hibernate that I've seen before include "go out in the snow?" , "go dormant" , "Sleep durning winter" , "Become dormant" , "Breathe in for long winter sleep" .)