Follow diamonds, say, in sportswear (9)
I believe the answer is:
tracksuit
'sportswear' is the definition.
(I've seen this before)
'follow diamonds say' is the wordplay.
'follow' becomes 'track' (I've seen this before).
'diamonds say' becomes 'suit' (diamonds is a playing card suit).
'track'+'suit'='TRACKSUIT'
'in' is the link.
(Other definitions for tracksuit that I've seen before include "Kit" , "athletic gear" , "Loose-fitting top and bottom for athlete warming up" , "Warm loose-fitting garment for exercising" , "Sportswear" .)