I have supper after ten? (6)
I believe the answer is:
iodine
'i' is the definition.
('iodine' can be a synonym of 'i')
'have supper after ten?' is the wordplay.
'have supper' becomes 'dine' (I've seen this before).
'after' says to put letters next to each other.
'ten?' becomes 'io'.
'dine' put after 'io' is 'IODINE'.
(Other definitions for iodine that I've seen before include "for therapeutic purposes" , "Element that can exist as a purple vapour" , "Tincture for cuts" , "Element existing in vapour and crystalline form" , "Antiseptic found in seaweed" .)