Bird swallows a little green man? (7)
I believe the answer is:
martian
'little green man?' is the definition.
(I've seen this before)
'bird swallows a' is the wordplay.
'bird' becomes 'martin' (martin is a kind of bird).
'swallows' is an insertion indicator.
'martin' placed around 'a' is 'MARTIAN'.
(Other definitions for martian that I've seen before include "Being from another planet" , "Invader in The War of the Worlds (H. G. Wells)" , "Extraterrestrial" , "Sci-fi alien" , "seeing little green man?" .)