Cool smart actress, one of a pair (7)
I believe the answer is:
hepburn
'actress one of a pair' is the definition.
(Katharine Hepburn or Audrey Hepburn)
'cool smart' is the wordplay.
'cool' becomes 'hep' (old-fashioned term for hip or cool).
'smart' becomes 'burn' (burning is a kind of smarting).
'hep'+'burn'='HEPBURN'
(Other definitions for hepburn that I've seen before include "Actress Katharine or Audrey" , "Audrey --, film actress" , "Audrey and Katherine, film stars" , "See 9" , "Audrey -; Katharine -, film actresses" .)