Tommy's quiet; possibly a sign of illness (7)
I believe the answer is:
symptom
'a sign of illness' is the definition.
(I've seen this before)
'tommy's quiet possibly' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'possibly' indicates anagramming the letters.
'tommys'+'p'='tommysp'
'tommysp' anagrammed gives 'SYMPTOM'.
(Other definitions for symptom that I've seen before include "Sign; indication" , "Indicator of disease" , "sign all's not well" , "Indication of illness" , "Sign of illness" .)