Irish centre in rugby jumper (4)
I believe the answer is:
erse
This might be a double definition.
'irish' is the first definition.
(old-fashioned term for the Irish language)
'centre in rugby jumper' is the second definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
(Other definitions for erse that I've seen before include "Celtic tongue" , "Kind of Gaelic for seer" , "Old name for Celtic languages" , "Old Gaelic for the seer" , "Irish Gaelic language" .)