Follow diamonds, say, in sporting attire (9)
I believe the answer is:
tracksuit
'sporting attire' is the definition.
'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' acts as a link.
(Other definitions for tracksuit that I've seen before include "Kit" , "Loose-fitting top and bottom for athlete warming up" , "athletic gear" , "Sportswear" , "Loose sports/casual wear" .)