Small vehicle to rent out is red (7)
I believe the answer is:
scarlet
'is red' is the definition.
(synonyms)
'small vehicle to rent out' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vehicle' becomes 'car' (car is a kind of vehicle).
'to rent out' becomes 'let' (I've seen this before).
's'+'car'+'let'='SCARLET'
(Other definitions for scarlet that I've seen before include "Fallen" , "Red colour of clarets" , "Vivid red" , "Bright red of clarets" , "Murderer" .)