Koeman's certainly in an unknown Dutch team (4)
I believe the answer is:
ajax
'dutch team' is the definition.
(AFC Ajax, Dutch football team)
'koeman's certainly in an unknown' is the wordplay.
'koeman's certainly' becomes 'ja' ('yes' in Dutch, language of Ronald Koeman).
'in' is an insertion indicator.
'an' becomes 'a'.
'unknown' becomes 'x' (algebra).
'a'+'x'='ax'
'ja' going within 'ax' is 'AJAX'.
(Other definitions for ajax that I've seen before include "Old war hero" , "Mythical Greek hero - Amsterdam football club" , "Cleaner" , "eleven from the Netherlands" , "side" .)