Charmingly old-fashioned designer carries one (6)
I believe the answer is:
quaint
'charmingly old-fashioned' is the definition.
(quaint means cutely old-fashioned)
'designer carries one' is the wordplay.
'designer' becomes 'Quint' (fashion designer Mary Quint).
'carries' indicates putting letters inside (inserted letters carried inside).
'one' becomes 'a' (a thing is one thing).
'quint' going around 'a' is 'QUAINT'.
(Other definitions for quaint that I've seen before include "Attractive, in an old-fashioned way" , "'Olde-worlde'" , "Charming" , "Curious" , "Attractively unusual or old-fashioned" .)