Flier in two arguments (7)
I believe the answer is:
sparrow
'flier' is the definition.
(sparrow is a kind of birdanimal)
'two arguments' is the wordplay.
'two arguments' can mean two replacements for 'argument'.
'argument' becomes 'spar' (I can't explain this - if you can you should give a lot more credence to this answer).
'argument' becomes 'row' (to row is to argue).
'spar'+'row'='SPARROW'
'in' is the link.
Can you help me to learn more?
(Other definitions for sparrow that I've seen before include "Little brown bird" , "Finch-like bird" , "House bird" , "(Formerly very) common British bird" , "bird produced" .)