The first to hire river (5)
I believe the answer is:
trent
'river' is the definition.
(I've seen this before)
'the first to hire' is the wordplay.
'the first' becomes 't' (1st letter of 'the').
'to hire' becomes 'rent' ('rent' can be a synonym of 'hire').
't'+'rent'='TRENT'
(Other definitions for trent that I've seen before include "Midlands banker" , "running water" , "Stoke's river" , "Nottingham's river" , "Chief river of the Midlands" .)