Elephant: black thing that flies! (5,3)
I believe the answer is:
jumbo jet
'thing that flies' is the definition.
(jumbo jets fly)
'elephant black' is the wordplay.
'elephant' becomes 'jumbo' (I've seen this before).
'black' becomes 'jet' (jet black is a kind of black).
'jumbo'+'jet'='JUMBO JET'
(Other definitions for jumbo jet that I've seen before include "Large plane" , "Flier" , "Owner of trunk" , "Large aeroplane" , "Large aircraft" .)