Get fitter calves, once exercised (10)
I believe the answer is:
convalesce
'get fitter' is the definition.
(to convalesce is to get better after illness)
'calves once exercised' is the wordplay.
'exercised' is an anagram indicator.
'calves'+'once'='calvesonce'
'calvesonce' is an anagram of 'CONVALESCE'.
(Other definitions for convalesce that I've seen before include "Improve" , "Recover after an illness" .)