See this key is inserted back to front (5)
I believe the answer is:
sight
'see' is the definition.
(thesaurus)
'this key is inserted back to front' is the wordplay.
'key' becomes 'g' (I've seen this in other clues).
'is inserted' is an insertion indicator.
'back to front' says the letters should be written in reverse.
'this' going around 'g' is 'thgis'.
'thgis' backwards is 'SIGHT'.
(Other definitions for sight that I've seen before include "Spectacle; aiming aid" , "Vista" , "A lot" , "a great deal" , "Glimpse" .)