Shabby races held in river (5)
I believe the answer is:
tatty
'shabby' is the definition.
('tatty' can be a synonym of 'shabby')
'races held in river' is the wordplay.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'held in' means one lot of letters goes inside another.
'river' becomes 'tay' (Scottish river).
'tt' put into 'tay' is 'TATTY'.
(Other definitions for tatty that I've seen before include "Neglected" , "Worn, scruffy" , "of inferior quality" , "In poor condition" , "Frayed" .)