Stone that gleams oddly? (3)
I believe the answer is:
gem
'stone' is the definition.
(gemstone)
'gleams oddly?' is the wordplay.
'oddly?' means one should take alternating letters (letters in odd-numbered positions).
The alternating letters of 'gleams' are 'GEM'.
'that' is the link.
(Other definitions for gem that I've seen before include "Valuable stone" , "perfect specimen" , "36 Across or any other jewel" , "A precious stone like a diamond or emerald etc" , "Diamond, ruby, or emerald" .)