A gem that shines only at night? (9)
I believe the answer is:
moonstone
'a gem' is the definition.
(moonstone is a kind of gem)
'shines only at night?' is the wordplay.
I cannot really understand how this works, but
'only' could be 'one' (synonyms) and 'one' is present in the answer.
'at' could be 'on' (location) and 'on' is found in the answer.
The remaining letters 'most' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'that' acts as a 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" .)