Restricting crossing river is causing strain (6)
I believe the answer is:
trying
'causing strain' is the definition.
(I've seen this before)
'restricting crossing river' is the wordplay.
'restricting' becomes 'tying' (tying is a kind of restricting).
'crossing' is an insertion indicator (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation).
'tying' placed around 'r' is 'TRYING'.
'is' acts as a link.
(Other definitions for trying that I've seen before include "Doing one's best - annoying!" , "Taxing" , "undertaking" , "Testing" , "Upsetting" .)