Recorded number on case for trainer's loose kit (9)
I believe the answer is:
tracksuit
'trainer's loose kit' is the definition.
(a tracksuit is a loose-fitting sports kit)
'recorded number on case' is the wordplay.
'recorded number' becomes 'track' (track can mean a recorded number or song).
'on' says to put letters next to each other.
'case' becomes 'suit' (synonyms).
'track'+'suit'='TRACKSUIT'
'for' is the link.
(Other definitions for tracksuit that I've seen before include "Sportswear" , "Loose-fitting top and bottom for athlete warming up" , "Loose sports/casual wear" , "athletic gear" , "Kit" .)