Sign is poor after treatment, having caught cold (7)
I believe the answer is:
scorpio
'sign' is the definition.
(sign of the zodiac)
'is poor after treatment having caught cold' is the wordplay.
'after treatment' indicates anagramming the letters (letters treated or processed in a new way).
'having caught' is an insertion indicator.
'cold' becomes 'C' (abbreviation, eg on taps).
'is'+'poor'='ispoor'
'ispoor' anagrammed gives 'sorpio'.
'sorpio' enclosing 'c' is 'SCORPIO'.
(Other definitions for scorpio that I've seen before include "7 [SIGN OF THE ZODIAC]" , "A heavenly body" , "Oct - Nov zodiac sign" , "Oct-Nov zodiac sign" , "Astrological sign" .)