Painter unexpectedly catching river turtle (8)
I believe the answer is:
terrapin
'turtle' is the definition.
(I know that terrapin is a type of turtle)
'painter unexpectedly catching river' is the wordplay.
'unexpectedly' indicates an anagram.
'catching' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation).
'painter' anagrammed gives 'terapin'.
'terapin' enclosing 'r' is 'TERRAPIN'.
(Other definitions for terrapin that I've seen before include "Water tortoise" , "freshwater creature" , "A reptile; a printer (anag.)" , "Web-footed turtle" , "Hardback" .)