Fashionable friend of Barbie in yellow (7)
I believe the answer is:
chicken
'yellow' is the definition.
(as in cowardly)
'fashionable friend of barbie' is the wordplay.
'fashionable' becomes 'chic' (chic means stylish or fashionable).
'friend of barbie' becomes 'ken' (I've seen this in another clue).
'chic'+'ken'='CHICKEN'
'in' acts as a link.
(Other definitions for chicken that I've seen before include "Cowardly - young fowl" , "daring game" , "Bird, coward (colloq.)" , "easily scared" , "Cowardly (colloq.)" .)