Improved our stamina in clinic (10)
I believe the answer is:
sanatorium
'clinic' is the definition.
(thesaurus)
'improved our stamina' is the wordplay.
'improved' is an anagram indicator.
'our'+'stamina'='ourstamina'
'ourstamina' anagrammed gives 'SANATORIUM'.
'in' acts as a link.
(Other definitions for sanatorium that I've seen before include "Convalescent hospital" , "health farm" , "convalescent home" , "Health resort" .)