Unknown on second team is eccentric (6)
I believe the answer is:
screwy
'eccentric' is the definition.
(thesaurus)
'unknown on second team' is the wordplay.
'unknown' becomes 'y' (algebra).
'on' says to put letters next to each other.
'second' becomes 's' (common abbreviation).
'team' becomes 'crew' (crew is a kind of team).
's'+'crew'='screw'
'y' put after 'screw' is 'SCREWY'.
'is' is the link.
(Other definitions for screwy that I've seen before include "cuckoo?" , "Off one's rocker" , "Slightly mad" , "like a prison officer?" , "Daft (with a thread?)" .)