The first to hire a river (5)
I believe the answer is:
trent
'a river' is the definition.
(River Trent)
'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 "Major English river" , "Burton's river" , "Crossword frame" , "Chief river of the Midlands" , "Midlands banker" .)