Every single sweetheart with a child (4)
I believe the answer is:
each
'every single' is the definition.
(I've seen this in another clue)
'sweetheart with a child' is the wordplay.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'with' says to put letters next to each other.
'child' becomes 'ch' (abbreviation for child).
'e'+'a'+'ch'='EACH'
(Other definitions for each that I've seen before include "Everyone separately" , "Every one of a number" , "Per capita, apiece" , "Singly, per capita" , "This way for a place" .)