Labels game on Sunday (4)
I believe the answer is:
tags
'labels' is the definition.
('tag' can be a synonym of 'label')
'game on sunday' is the wordplay.
'game' becomes 'tag' (tag is a kind of game**).
'on' is a charade indicator (letters next to each other).
'sunday' becomes 's'.
'tag'+'s'='TAGS'
(Other definitions for tags that I've seen before include "They are able to identify" , "Labels; quotations" , "trite Latin quotations" .)