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' is an anagram indicator (the letters are active/moving).
'in' indicates putting letters inside.
'west' becomes 'W' (abbreviation).
'and' means one lot of letters go next to another.
'north' becomes 'N' (abbreviation).
'very' with letters rearranged gives 'yver'.
'w'+'n'='wn'
'yver' going 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" .)