The first to hire river (5)
I believe the answer is:
trent
'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' (synonyms).
'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" .)