Reserved yet looking very fit (8-2)
I believe the answer is:
buttoned-up
'reserved' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'yet looking very fit' is the wordplay.
I cannot really see how this works, but
'yet' could be 'but' (synonyms) and 'but' is found in the answer.
'fit' could be 'toned' (I've seen this in another clue) and 'toned' is found in the answer.
The remaining letters 'up' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for buttoned-up that I've seen is " Taciturn (slang)".)