See turning this key in it (5)
I believe the answer is:
sight
'see' is the definition.
(thesaurus)
'turning this key in it' is the wordplay.
'turning' says the letters should be written backwards.
'key' becomes 'g' (I've seen this in other clues).
'in it' is an insertion indicator.
'this' placed around 'g' is 'thgis'.
'thgis' written backwards gives 'SIGHT'.
(Other definitions for sight that I've seen before include "Power of vision" , "Notice" , "The ability to see - spectacle" , "good deal" , "A lot" .)