Be inclined to admit designer, finally, is in fashion (5)
I believe the answer is:
trend
'fashion' is the definition.
(I've seen this before)
'be inclined to admit designer finally' is the wordplay.
'be inclined' becomes 'tend' (synonyms).
'to admit' is an insertion indicator.
'finally' suggests the final letters.
The final letter of 'designer' is 'r'.
'tend' going around 'r' is 'TREND'.
'is in' is the link.
(Other definitions for trend that I've seen before include "General direction in which something is developing" , "General direction of movement" , "Med. port, taken by Crusaders" , "be all over Twitter, Instagram, etc" , "Current fashion" .)