Crossing river is big problem for marsupial (7)
I believe the answer is:
opossum
'for marsupial' is the definition.
(opossum is a kind of marsupial)
'crossing river is big problem' is the wordplay.
'crossing' is an insertion indicator (to cross can mean to go over or around).
'river' becomes 'Po' (Italian river).
'is big' becomes 'os' (abbreviation for oversize).
'problem' becomes 'sum' (arithmetic problem).
'po' going inside 'os' is 'opos'.
'opos'+'sum'='OPOSSUM'
(Other definitions for opossum that I've seen before include "Tree climber" , "American native" , "Native of America" , "Small arboreal marsupial" , "American and Australian marsupial, often with the first letter dropped" .)