Hibernating snake securing shelter (6)
I believe the answer is:
asleep
'hibernating' is the definition.
(similar in meaning)
'snake securing shelter' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'securing' means one lot of letters goes inside another.
'shelter' becomes 'lee' (as in lee side, the side sheltered from wind).
'asp' placed around 'lee' is 'ASLEEP'.
(Other definitions for asleep that I've seen before include "Please be at rest (6)." , "In the arms of Hypnos" , "In the arms fo Morpheus" , "Slumbering" , "like a kipper" .)