Extremely good-looking NI footballer keeps playing there (8)
I believe the answer is:
bonniest
'playing there' is the definition.
I can't judge whether this defines the answer.
'extremely good-looking ni footballer keeps' is the wordplay.
'extremely good looking' becomes 'on' (I can't justify this - if you can you should give a lot more credence to this answer).
'footballer' becomes 'Best' (George Best, o946-n005).
'keeps' indicates putting letters inside (inserted letters are kept inside).
'on'+'ni'='onni'
'onni' going within 'best' is 'BONNIEST'.
Can you help me to learn more?
(Another definition for bonniest that I've seen is " Most healthy-looking baby".)