Back crossing river to west, further from common? (5)
I believe the answer is:
rarer
'further from common?' is the definition.
(less common)
'back crossing river to west' is the wordplay.
'back' becomes 'rear' (something's rear is its back).
'crossing' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'to west' shows that the letters should be reversed in order (the letters are written right to left).
'rear' enclosing 'r' is 'rerar'.
'rerar' written backwards gives 'RARER'.
(Other definitions for rarer that I've seen before include "Thinner" , "less easy to find" , "Less frequent" , "More unusual" , "Stranger" .)