Those eventually attacked, lowering heads, duck (4)
I believe the answer is:
teal
'duck' is the definition.
(I know that teal is a type of duck)
'those eventually attacked lowering heads' is the wordplay.
'heads' says to take the initial letters.
The first letters of 'those eventually attacked lowering' is 'TEAL'.
(Other definitions for teal that I've seen before include "Variety of duck" , "Variety of small ducks" , "Type of duck" , "Small river duck" , "Species of duck" .)