Parent hugs sweetheart who is unresponsive (4)
I believe the answer is:
dead
'is unresponsive' is the definition.
(I've seen this before)
'parent hugs sweetheart' is the wordplay.
'parent' becomes 'dad' (dad can informally mean father).
'hugs' means one lot of letters goes inside another.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'dad' going around 'e' is 'DEAD'.
'who' acts as a link.
(Other definitions for dead that I've seen before include "Lifeless" , "Passed on" , "Gone to meet one's Maker" , "Six feet under" , "No longer alive" .)