Monster very active in West and North (6)
I believe the answer is:
wyvern
'monster' is the definition.
(I've seen this before)
'very active in west and north' is the wordplay.
'active' indicates an anagram (the letters are active/moving).
'in' means one lot of letters goes inside another.
'west' becomes 'W' (abbreviation).
'and' means one lot of letters go next to another.
'north' becomes 'N' (abbreviation).
'very' anagrammed gives 'yver'.
'w'+'n'='wn'
'yver' placed into 'wn' is 'WYVERN'.
(Other definitions for wyvern that I've seen before include "Heraldic dragon with two legs and two wings" , "Winged two-legged dragon (heraldry)" , "Dragon with wings in medieval heraldry" , "Fire-breathing dragon with a snake's tail" , "In heraldry, a dragon with wings, two legs and a barbed tail" .)