Secluded spot (4)
'secluded spot' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nook that I've seen before include "cosy retreat" , "Alcove" , "Cranny or niche" , "Corner or small recess" , "Cosy corner" .)
(Other definitions for glen that I've seen before include "Highland valley" , "Scottish or Irish valley" , "Scottish mountain valley" , "Narrow valley with a stream" , "Depression in Scotland" .)