Snake outside shelter having forty winks (6)
I believe the answer is:
asleep
'having forty winks?' is the definition.
(I've seen this before)
'snake outside shelter' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'outside' means one lot of letters goes inside another (some letters go outside others).
'shelter' becomes 'lee' (as in lee side, the side sheltered from wind).
'asp' enclosing 'lee' is 'ASLEEP'.
(Other definitions for asleep that I've seen before include "(Of a limb) numb" , "like a kipper" , "Having forty winks" , "Having nodded off" , "coiled up?" .)