Number of fashion items in Mary Quant's era (7)
I believe the answer is:
sixties
'mary quant's era' is the definition.
(reference to 60's fashion designer Mary Quant)
'number of fashion items' is the wordplay.
'number of' becomes 'six'.
'fashion items' becomes 'ties' (items of clothing).
'six'+'ties'='SIXTIES'
'in' is the link.
(Other definitions for sixties that I've seen before include "time to retire?" , "some years?" , "The Age of Aquarius" .)