Count flies besieging a bird (6)
I believe the answer is:
toucan
'bird' is the definition.
(toucan is a kind of bird)
'count flies besieging a' is the wordplay.
'flies' indicates an anagram.
'besieging' is an insertion indicator.
'count' with letters rearranged gives 'toucn'.
'toucn' placed around 'a' is 'TOUCAN'.
(Other definitions for toucan that I've seen before include "Tropical bird with large bill" , "Large-billed tropical bird" , "Bird with large colourful beak" , "arboreal dweller" , "Coloured tropical bird with a large beak" .)