Overdue, headless and happy (6)
I believe the answer is:
elated
'happy' is the definition.
('elated' can be similar in meaning to 'happy')
'overdue headless' is the wordplay.
'overdue' becomes 'belated' (I've seen this before).
'headless' means to remove the first letter.
'belated' with its initial letter taken off is 'ELATED'.
'and' is the link.
(Other definitions for elated that I've seen before include "Very pleased, quite high" , "In an enraptured state" , "Jubilant, overjoyed" , "Over the moon" , "Filled with joy and pride" .)