It can't fly in your heading (4)
I believe the answer is:
rhea
'it can't fly' is the definition.
(I've seen this before)
'in your heading' is the wordplay.
'in' indicates a hidden word.
'RHEA' can be found hidden inside 'your heading'.
(Other definitions for rhea that I've seen before include "Flightless bird -- satellite of Saturn" , "Moon" , "Big bird" , "South American bird resembling the ostrich" , "China grass" .)