Lake encircled by a sheltered walkway (5)
I believe the answer is:
'walkway' is the definition.
'lake encircled by a sheltered' is the wordplay.
'lake' becomes 'l'.
'encircled by' indicates putting letters inside.
'sheltered' becomes 'lee' (as in lee side, the side sheltered from wind).
'l' inserted within 'alee' is 'ALLEE'.
(Other definitions for allee that I've seen before include "Path with trees" , "Parisian walk" , "Pleasant walk" , "part of park" , "garden path?" .)