Exceptional pressure of the joints (10)
I believe the answer is:
particular
'exceptional' is the definition.
(synonyms)
'pressure of the joints' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'of the joints' becomes 'articular' (I've seen this in another clue).
'p'+'articular'='PARTICULAR'
(Other definitions for particular that I've seen before include "Special" , "Specific" , "Fog over London" , "Fastidious; individual" , "Nice" .)