Dozing snake has secured shelter (6)
I believe the answer is:
asleep
'dozing' is the definition.
(similar in meaning)
'snake has secured shelter' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'has secured' is an insertion indicator.
'shelter' becomes 'lee' (as in lee side, the side sheltered from wind).
'asp' going around 'lee' is 'ASLEEP'.
(Other definitions for asleep that I've seen before include "Having nodded off" , "Slumbering" , "Resting unconscious" , "In the arms of Hypnos" , "Dormant, inactive" .)