Bird beginning to peck sweetheart (6)
I believe the answer is:
plover
'bird' is the definition.
(plover is a kind of bird)
'beginning to peck sweetheart' is the wordplay.
'beginning to' indicates taking the first letters.
'sweetheart' becomes 'lover' (sweetheart can mean a lover).
The initial letter of 'peck' is 'p'.
'p'+'lover'='PLOVER'
(Other definitions for plover that I've seen before include "Shore bird with long pointed wings" , "Short-billed shore bird" , "Wading bird, such as a peewit" , "Lapwing-like bird" , "Wader" .)