Say about the old girl's monocle (8)
I believe the answer is:
eyeglass
'monocle' is the definition.
('eyeglass' can be a synonym of 'monocle')
'say about the old girl's' is the wordplay.
'say' becomes 'eg'.
'about' is an insertion indicator.
'the old' becomes 'ye'.
'girl' becomes 'lass' (lass is a kind of girl**).
'eg' placed around 'ye' is 'eyeg'.
'eyeg'+'lass'='EYEGLASS'
(Other definitions for eyeglass that I've seen before include "Monocle, or spectacles in plural" , "Lens for aiding defective vision" , "Aid to vision" .)