See good girl in monocle (8)
I believe the answer is:
eyeglass
'monocle' is the definition.
('eyeglass' can be a synonym of 'monocle')
'see good girl' is the wordplay.
'see' becomes 'eye' (I've seen this before).
'good girl' becomes 'glass' (I've seen this in other clues).
'eye'+'glass'='EYEGLASS'
'in' is the link.
(Other definitions for eyeglass that I've seen before include "Aid to vision" , "Lens for aiding defective vision" , "Monocle, or spectacles in plural" .)