Knocked over giant, thus (4)
I believe the answer is:
ergo
'thus' is the definition.
(thesaurus)
'knocked over giant' is the wordplay.
'knocked over' says the letters should be written backwards.
'giant' becomes 'ogre' (ogre is a kind of giant).
'ogre' reversed gives 'ERGO'.
(Other definitions for ergo that I've seen before include "Hence" , "Therefore - what a goer" , "so philosopher wrote in Latin" , "as a consequence" , "Therefore (Latin)" .)