Flightless bird trapped by Ledger (Heath) (4)
I believe the answer is:
rhea
'flightless bird' is the definition.
(I know that rhea is a type of flightless bird)
'trapped by ledger heath' is the wordplay.
'trapped' says the answer is hidden in the clue.
'RHEA' is hidden within 'by ledger heath'.
(Other definitions for rhea that I've seen before include "Greek goddess of fertility - running bird" , "China grass" , "Fast runner" , "S. Amer. flightless bird" , "South American bird resembling the ostrich" .)