Stylish companion of Barbie is a coward (7)
I believe the answer is:
chicken
'a coward' is the definition.
(I've seen this in another clue)
'stylish companion of barbie' is the wordplay.
'stylish' becomes 'chic' (similar in meaning).
'companion of barbie' becomes 'ken' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'chic'+'ken'='CHICKEN'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for chicken that I've seen before include "easily scared" , "Informally cowardly like domestic fowl" , "In check (anag.)" , "Domestic bird - frightened" , "Young hen" .)