Giant bird with origins in Arabian Nights (5)
I believe the answer is:
titan
'giant' is the definition.
(I know this)
'bird with origins in arabian nights' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'with' means one lot of letters go next to another.
'origins in' says to take the initial letters (the first letter is the origin of the word).
The first letters of 'arabian nights' is 'an'.
'tit'+'an'='TITAN'
(Other definitions for titan that I've seen before include "heavyweight?" , "Person of great importance" , "Saturn's moon" , "Person of great size" , "Moon of Saturn visited by the Huygens probe" .)