Create green den when rambling (8)
I believe the answer is:
engender
'create' is the definition.
(engendering is a kind of creating)
'green den when rambling' is the wordplay.
'when rambling' is an anagram indicator (I've seen this in other clues).
'green'+'den'='greenden'
'greenden' is an anagram of 'ENGENDER'.
(Other definitions for engender that I've seen before include "Give rise to; beget" , "Father" , "Precipitate" , "Give rise to, cause" , "Ned Green could breed" .)