Actress’s old boast recalled (5)
I believe the answer is:
garbo
'actress's' is the definition.
(I've seen this before)
'old boast recalled' is the wordplay.
'old' becomes 'o' (abbreviation).
'boast' becomes 'brag' (synonyms).
'recalled' is a reversal indicator (to recall can mean to bring something back).
'o'+'brag'='obrag'
'obrag' back-to-front is 'GARBO'.
(Other definitions for garbo that I've seen before include "film star? Yes and no" , "Greta -, Swed. actress" , "film-star" , "Reclusive film star" , "Famous film actress" .)