Star cooped up, putting on some weight (9)
I believe the answer is:
pentagram
'star' is the definition.
(pentagram is a kind of star)
'cooped up putting on some weight' is the wordplay.
'cooped up' becomes 'pent' (both can mean enclosed).
'putting on' means one lot of letters go next to another.
'some' becomes 'a'.
'weight' becomes 'gram' (I have seen 'Metric weight ' mean 'gram' so perhaps 'weight' could also mean 'gram').
'pent'+'a'+'gram'='PENTAGRAM'
(Other definitions for pentagram that I've seen before include "Five-pointed star" , "Five pointed star" , "magical symbol" , "Star-shaped figure" , "Mystical symbol" .)