First or last of smiles, if not gloomy (7)
I believe the answer is:
sunless
'gloomy' is the definition.
(I've seen this in another clue)
'first or last of smiles if not' is the wordplay.
I cannot quite understand how this works, but
'if' could be 'unless' (I've seen this before) and 'unless' is located in the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sunless that I've seen before include "Unlit by solar light" , "Offering nothing for a tanner" , "Cloudy" , "Dull" , "dim, certainly not bright" .)