As there’s rain, are driven back (4)
I believe the answer is:
rear
'back' is the definition.
('rear' can be a synonym of 'back')
'as there's rain are driven' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'are' is 'rea' which is within the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rear that I've seen before include "Buttocks - rise up" , "Bring up and care for" , "Of horse, rise up on hind legs" , "Rump" , "Rare at the back" .)