The head ducks as well (3)
I believe the answer is:
too
'as well' is the definition.
('too' can be a synonym of 'as well')
'the head ducks' is the wordplay.
'head' indicates taking the first letters.
'ducks' becomes 'oo' (I've seen this in other clues).
The initial letter of 'the' is 't'.
't'+'oo'='TOO'
(Other definitions for too that I've seen before include "To a greater than desirable extent" , "Excessively" , "As well, also" , "Unduly" , "Also; overly" .)