Stalwart sore after playing musical instrument (8)
I believe the answer is:
resolute
'stalwart' is the definition.
(similar in meaning)
'sore after playing musical instrument' is the wordplay.
'after playing' is an anagram indicator.
'musical instrument' becomes 'lute' (lute is a kind of musical instrument).
'sore' is an anagram of 'reso'.
'reso'+'lute'='RESOLUTE'
(Other definitions for resolute that I've seen before include "Admirably purposeful" , "Determined, firm" , "Firmly determined" , "'Firm, unhesitating (8)'" , "Firm in purpose" .)