One in fashion pages is very modern (5-3)
I believe the answer is:
space-age
'very modern' is the definition.
(I've seen this before)
'one in fashion pages' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'in' means one lot of letters goes inside another.
'fashion' indicates anagramming the letters.
'pages' with letters rearranged gives 'spage'.
'ace' inserted into 'spage' is 'SPACE-AGE'.
'is' is the link.
(Other definitions for space-age that I've seen before include "Ultra-modern" , "Futuristic" .)