Couple joined up under Eye, it's said (4)
I believe the answer is:
item
'couple' is the definition.
(a romantic couple are 'an item')
'joined up under eye it's said' is the wordplay.
'joined' becomes 'met' (meeting is a kind of joining).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'under' means one lot of letters go next to another (in a down clue, some letters go below others).
'it's said' indicates a 'sounds like' (homophone) clue.
'eye' is a homophone of 'i'.
'met' reversed gives 'tem'.
'tem' after 'i' is 'ITEM'.
(Other definitions for item that I've seen before include "Boyfriend and girlfriend" , "Loving couple" , "Part of a list" , "Record" , "couple making out {TIME}..." .)