A monstrous giant makes the queen go back (4)
I believe the answer is:
ogre
'a monstrous giant' is the definition.
(I've seen this before)
'the queen go back' is the wordplay.
'the queen' becomes 'ER' (Elizabeth Regina).
'back' says the letters should be written in reverse.
'er'+'go'='ergo'
'ergo' back-to-front is 'OGRE'.
'makes' acts as a link.
(Other definitions for ogre that I've seen before include "Terrifying giant" , "Jack's giant perhaps" , "Evil giant" , "ugly one" , "Giant who eats people" .)