Climber using narrow defile in crossing old river? (13)
I believe the answer is:
passionflower
'climber' is the definition.
(type of climbing plant)
'narrow defile in crossing old river?' is the wordplay.
'narrow defile' becomes 'pass' (a defile is a type of gorge or path).
'crossing' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'river?' becomes 'flower' (a river flows).
'in' enclosing 'o' is 'ion'.
'pass'+'ion'+'flower'='PASSIONFLOWER'
'using' is the link.
(Other definitions for passionflower that I've seen before include "Climbing plant, Passiflora" , "Tropical climbing plant (granadilla)" .)