A gem that shines only at night? (9)
I believe the answer is:
moonstone
'a gem' is the definition.
(I know that moonstone is a type of transparent gem)
'shines only at night?' is the wordplay.
I cannot quite understand how this works, but
'only' could be 'one' (synonyms) and 'one' is located in the answer.
'at' could be 'on' (location) and 'on' is located in the answer.
The remaining letters 'most' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'that' is the link.
Can you help me to learn more?
(Other definitions for moonstone that I've seen before include "something shiny" , "feldspar" , "Semi-precious gem described in monotones" .)