Tiresome person quietly suffering (4)
I believe the answer is:
pill
'tiresome person' is the definition.
(I've seen this in another clue)
'quietly suffering' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'suffering' becomes 'ill' (suffering with illness).
'p'+'ill'='PILL'
(Other definitions for pill that I've seen before include "Pellet of medicine" , "Medication" , "Dose of medicine" , "Medicinal tablet" , "may be the cure" .)