Monstrous creature, so should be retired (4)
I believe the answer is:
ogre
'monstrous creature' is the definition.
(I've seen this before)
'so should be retired' is the wordplay.
'so' becomes 'ergo' (synonyms).
'should be retired' says the letters should be written backwards (retire can mean to go back).
'ergo' backwards is 'OGRE'.
(Other definitions for ogre that I've seen before include "Fairy tale giant" , "man-eater" , "Monstrous person" , "character in 'Puss in Boots'" , "Evil giant" .)