Quietly nag about painful emotion (4)
I believe the answer is:
pang
'painful emotion' is the definition.
(I've seen this before)
'quietly nag about' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'about' indicates an anagram.
'nag' anagrammed gives 'ang'.
'p'+'ang'='PANG'
(Other definitions for pang that I've seen before include "Sudden sharp pain" , "Sharp emotional pain" , "Mental pain or distress" , "Stab of pain" , "Sharp pain, of hunger or emotion perhaps" .)