Giant bird on land, wingless (5)
I believe the answer is:
titan
'giant' is the definition.
(titans are giant mythological creatures)
'bird on land wingless' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'wingless' says to take the centre (the wings or sides of the letters are removed).
The centre of 'land' is 'an'.
'tit'+'an'='TITAN'
(Other definitions for titan that I've seen before include "Mythological giant" , "immense figure" , "Old giant" , "Taint (anag.)" , "Colossus" .)