Met up on one particular (4)
I believe the answer is:
item
'particular' is the definition.
(I know that particular can be written as item)
'met up on one' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'on' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'met' in reverse letter order is 'tem'.
'tem' put after 'i' is 'ITEM'.
(Other definitions for item that I've seen before include "date" , "Object; piece" , "couple linked" , "newspaper article" , "Single article on a list" .)