Heard dull person describing animal (4)
I believe the answer is:
boar
'animal' is the definition.
(I've seen this before)
'heard dull person' is the wordplay.
'heard' shows a homophone (sound like).
'dull person' becomes 'bore' (I've seen this before).
'bore' sounds like 'BOAR'.
'describing' is the link.
(Other definitions for boar that I've seen before include "Resting place for birds or bats" , "Wild pig sounds tedious" , "A male pig, sounds tedious" , "Uncastrated male hog" , "Wild animal, sounds like drill" .)