Old film star seen in dress circle (5)
I believe the answer is:
'old film star' is the definition.
The definition suggests a singular noun which matches the answer.
'dress circle' is the wordplay.
'dress' becomes 'garb' (synonyms).
'circle' becomes 'O' (O is a circle).
'seen in' acts as a link.
Can you help me to learn more?
(Other definitions for garbo that I've seen before include "film-star", "Swedish-born actress", "Greta -, Swed. actress", "film star? Yes and no", "Famous film actress".)