Blair casually departs, looking well fit (5)
I believe the answer is:
toned
'looking well fit' is the definition.
'toned' can be an answer for 'fit' (I've seen this in another clue). I'm not certain of the 'looking well' bit.
'blair casually departs' is the wordplay.
'blair casually' becomes 'tone' (I can't explain this - if you can you should give a lot more credence to this answer).
'departs' becomes 'd' (common abbreviation e.g. train timetables).
'tone'+'d'='TONED'
Can you help me to learn more?
(Other definitions for toned that I've seen before include "In good condition (of muscles or skin)" , "pitched just right" , "Muscular" , "Noted (anag.)" , "Braced" .)