Heraldic monster with very silly name (6)
I believe the answer is:
wyvern
'heraldic monster' is the definition.
(I've seen this before)
'with very silly name' is the wordplay.
'with' becomes 'w' (abbreviation**).
'silly' is an anagram indicator.
'name' becomes 'n' (common abbreviation e.g. on forms).
'very' is an anagram of 'yver'.
'w'+'yver'+'n'='WYVERN'
(Other definitions for wyvern that I've seen before include "Heraldic beast" , "1950s Vauxhall model" , "Fire-breathing dragon with a snake's tail" , "Dragon with wings in medieval heraldry" , "Heraldic device" .)