Bird turned up, bill on head — fancy dress? (7)
I believe the answer is:
costume
'fancy dress?' is the definition.
(fancy dress is a kind of costume)
'bird turned up bill on head' is the wordplay.
'bird' becomes 'emu' (emu is a kind of bird).
'turned up' says the letters should be written in reverse.
'bill' becomes 'cost'.
'on head' says to put letters next to each other.
'emu' written backwards gives 'ume'.
'ume' put after 'cost' is 'COSTUME'.
(Other definitions for costume that I've seen before include "(Historic etc) dress" , "ensemble" , "Rig" , "Theatrical clothes" , "Duds" .)