Tommy's upset about parking warning sign (7)
I believe the answer is:
symptom
'warning sign' is the definition.
'tommy's upset about parking' is the wordplay.
'upset' indicates anagramming the letters.
'about' indicates putting letters inside.
'parking' becomes 'p'.
'tommys' is an anagram of 'symtom'.
'symtom' placed around 'p' is 'SYMPTOM'.
(Other definitions for symptom that I've seen before include "Indicator of disease" , "sign all's not well" , "Prodrome" , "It alerts one to illness" , "Feature of a disease" .)