Boast about old star (5)
I believe the answer is:
'star' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?
'boast about old' is the wordplay.
'boast' becomes 'brag' (synonyms).
'about' is a reversal indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'brag' in reverse letter order is 'garb'.
Can you help me to learn more?
(Other definitions for garbo that I've seen before include "Swedish film actress Greta" , "Famous film actress" , "Reclusive film star" , "Greta -, Swed. actress" , "Swedish film star who wanted to be alone" .)