Spotted animal leaving Island’s river by mistake (5)
I believe the answer is:
gaffe
'mistake' is the definition.
(gaffe is a kind of mistake)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for gaffe that I've seen before include "embarrassing mistake" , "Remark one wishes unmade" , "Blundering remark" , "Indiscretion" , "A foolish error" .)