Marsupial has big problem crossing river (7)
I believe the answer is:
opossum
'marsupial' is the definition.
(opossum is a kind of marsupial)
'big problem crossing river' is the wordplay.
'big' becomes 'os'.
'problem' becomes 'sum' (arithmetic problem).
'crossing' indicates putting letters inside.
'river' becomes 'po' (Italian river).
'os'+'sum'='ossum'
'ossum' going around 'po' is 'OPOSSUM'.
'has' is the link.
(Other definitions for opossum that I've seen before include "Small arboreal marsupial" , "Small Australian marsupial" , "Thickly furred marsupial" , "Tree climber" , "American native" .)