Setter heard vessel getting slap on the lid (8)
I believe the answer is:
eyeliner
'slap on the lid' is the definition.
(slap can mean makeup - on eyelid)
'setter heard vessel getting' is the wordplay.
'setter' becomes 'I' (the person that set the crossword).
'heard' shows a homophone (sound like).
'vessel' becomes 'liner' (liner is a kind of vessel).
'getting' is a charade indicator (letters next to each other).
'i' sounds like 'eye'.
'eye'+'liner'='EYELINER'
(Other definitions for eyeliner that I've seen before include "Makeup for the peepers" , "Cosmetic pencil" , "it draws attention to the viewer" , "highlight for look-out" , "instrument for enhancing lamps" .)