Marine creature reportedly saw a taxi crossing river (6,4)
I believe the answer is:
spider crab
'marine creature reportedly' is the definition.
I can't tell whether this defines the answer.
'saw a taxi crossing river' is the wordplay.
'saw' becomes 'spider' (I am not sure about this - if you are sure you should believe this answer much more).
'a taxi' becomes 'cab' ('cab' can be a synonym of 'taxi').
'crossing' means one lot of letters goes inside another.
'river' becomes 'r'.
'cab' enclosing 'r' is 'crab'.
'spider'+'crab'='SPIDER-CRAB'
(Other definitions for spider crab that I've seen before include "Crustacean with long thin legs" , "little creature" , "Aquatic creature" , "Type of crustacean" , "shellfish" .)