Flier, black one, blocks main road (7)
I believe the answer is:
seabird
'flier' is the definition.
(seabird is a kind of birdanimal)
'black one blocks main road' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'one' becomes 'i' (Roman numeral).
'blocks' is an insertion indicator (the inside letters form a 'blockage').
'main' becomes 'sea' (synonyms).
'road' becomes 'rd' (abbreviation).
'b'+'i'='bi'
'sea'+'rd'='seard'
'bi' inserted within 'seard' is 'SEABIRD'.
(Other definitions for seabird that I've seen before include "Marine creature" , "gull, say" , "A gull, for instance" , "Gull perhaps" , "Gull, for example" .)