Time to hire a river (5)
I believe the answer is:
trent
'a river' is the definition.
(River Trent)
'time to hire' is the wordplay.
'time' becomes 't' (abbreviation).
'to hire' becomes 'rent' (synonyms).
't'+'rent'='TRENT'
(Other definitions for trent that I've seen before include "River through Newark" , "Chief river of the Midlands" , "Midlands banker" , "Crossword frame" , "River at Stoke and Burton" .)