Lifeless old man embraces sweetheart (4)
I believe the answer is:
'lifeless' is the definition.
('dead' can be a synonym of 'lifeless')
'old man embraces sweetheart' is the wordplay.
'old man' becomes 'dad' ('dad' can be a synonym of 'old man').
'embraces' indicates putting letters inside.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'dad' placed around 'e' is 'DEAD'.
(Other definitions for dead that I've seen before include "Stiff - bereft of life" , "Late; exact" , "Gone to meet one's Maker" , "Passed on" , "dull" .)