Fashion designer pinning first of gems on a knitted garment (8)
I believe the answer is:
cardigan
'knitted garment' is the definition.
(I've seen this before)
'fashion designer pinning first of gems on a' is the wordplay.
'fashion designer' becomes 'cardin' (I've seen this before).
'pinning' is an insertion indicator.
'first of gems' becomes 'g' (1st letter of 'gems').
'on' means one lot of letters go next to another.
'g'+'a'='ga'
'cardin' placed around 'ga' is 'CARDIGAN'.
(Other definitions for cardigan that I've seen before include "One making ridiculous charge" , "Knitted garment" , "Warmer" , "See 25" , "corgi" .)