Jewel keeps winking, perhaps it's a kind of illusion (7)
I believe the answer is:
optical
'it's a kind of illusion' is the definition.
(optical illusion)
'jewel keeps winking' is the wordplay.
'jewel' becomes 'opal' (I've seen this before).
'keeps' is an insertion indicator.
'winking' becomes 'tic' (someone may wink as a tic).
'opal' placed around 'tic' is 'OPTICAL'.
'perhaps' acts as a link.
(Other definitions for optical that I've seen before include "Topical (anag.)" , "Working with light" , "Using visible light" , "To do with sight, perhaps an illusion" , "Concerning sight" .)