Couple definitely disheartened on a date (4)
I believe the answer is:
dyad
'couple' is the definition.
('dyad' can be a synonym of 'couple')
'definitely disheartened on a date' is the wordplay.
'disheartened' means to remove the middle letters.
'on' says to put letters next to each other.
'a date' becomes 'ad' (AD is an example).
'definitely' with its middle removed is 'dy'.
'dy'+'ad'='DYAD'
(Other definitions for dyad that I've seen before include "is in element" , "Couple" , "Pair" , "Something in two parts" .)