Partner shows fever after cello playing (9)
I believe the answer is:
colleague
'partner' is the definition.
(I've seen this before)
'fever after cello playing' is the wordplay.
'fever' becomes 'ague' (I've seen this in another clue).
'after' is a charade indicator (letters next to each other).
'playing' is an anagram indicator.
'cello' anagrammed gives 'colle'.
'ague' put after 'colle' is 'COLLEAGUE'.
'shows' acts as a link.
(Other definitions for colleague that I've seen before include "China, perhaps" , "Fellow worker" , "Business co-worker" , "Person with whom one works" , "Associate" .)