Most suitable gem for tiny fellow to wear (7)
I believe the answer is:
optimal
'most suitable' is the definition.
(optimal means the best)
'gem for tiny fellow to wear' is the wordplay.
I cannot really understand how this works, but
'fellow' could be 'tim' (I have seen 'Little fellow' mean 'tim' so perhaps 'fellow' could also mean 'tim') and 'tim' is located in the answer.
'gem' could be 'opal' (opal is a kind of gem) and 'opal' is found in the remaining letters.
No letters remain.
This may be the basis of clue (or it may be nonsense).
(Other definitions for optimal that I've seen before include "Am pilot being at best" , "Plot aim (anag) - best" , "Most desirable" , "Ideal" , "Best possible" .)