Indulgence of sweetheart in restaurant (5)
I believe the answer is:
treat
'indulgence' is the definition.
(to indulge yourself is to treat yourself)
'sweetheart in restaurant' is the wordplay.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'in' indicates putting letters inside.
'restaurant' becomes 'trat' (I've seen this in other clues).
'e' going into 'trat' is 'TREAT'.
'of' acts as a link.
(Other definitions for treat that I've seen before include "A nice surprise" , "Entertain - minister - free entertainment" , "Source of gratification" , "Give medical care or free meal to" , "Practise medicine" .)