Bird getting into your head (4)
I believe the answer is:
rhea
'bird' is the definition.
(I know that rhea is a type of flightless bird)
'into your head' is the wordplay.
'into' indicates the answer is hidden within the clue.
'RHEA' can be found hidden inside 'your head'.
'getting' acts as a link.
(Other definitions for rhea that I've seen before include "Large S. American bird" , "Fertility goddess in ancient Greek mythology" , "Moon" , "Fast-running flightless bird" , "Large flightless bird" .)