One or two fliers? (7)
I believe the answer is:
titlark
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'two fliers?' is the wordplay.
'two fliers?' means two replacements for 'flier'.
'flier' becomes 'tit' (tit is a kind of birdanimal).
'flier' becomes 'lark' (lark is a kind of birdanimal).
'tit'+'lark'='TITLARK'
'or' acts as a link.
(Other definitions for titlark that I've seen before include "bird" , "One flying" , "Magic lamp possessor" , "Meadow pipit" , "songbird" .)