Monster on the retreat in 'Mother Goose' (4)
I believe the answer is:
ogre
'monster' is the definition.
('ogre' can be a synonym of 'monster')
'the retreat in mother goose' is the wordplay.
'the retreat' says the letters should be written in reverse.
'in' indicates the answer is hidden within the clue.
'ergo' is hidden within 'mother goose'.
'ergo' back-to-front is 'OGRE'.
'on' is the link.
(Other definitions for ogre that I've seen before include "Human-eating giant" , "Terrifying person" , "Monstrous creature" , "Brute" , "Giant of folklore who liked to eat humans" .)