Shiner causes squint oddly (3)
I believe the answer is:
sun
'shiner' is the definition.
(I've seen this in another clue)
'squint oddly' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'squint' are 'SUN'.
'causes' acts as a link.
(Other definitions for sun that I've seen before include "Source of daylight" , "Source of light and heat" , "Get a tan" , "Helios, in Greek myth" , "Dawn riser" .)