See soldiers crossing island in French river (5)
I believe the answer is:
loire
'french river' is the definition.
(River Loire)
'see soldiers crossing island' is the wordplay.
'see' becomes 'lo' (archaic exclamation meaning 'look!').
'soldiers' becomes 're' (Royal Engineers).
'crossing' is an insertion indicator (to cross can mean to go over or around).
'island' becomes 'i' (geographical abbreviation).
'lo'+'re'='lore'
'lore' enclosing 'i' is 'LOIRE'.
'in' acts as a link.
(Other definitions for loire that I've seen before include "flower found amid vineyards" , "part of France" , "Wine region" , "France's longest river" , "French runner" .)