Unknown on second team is eccentric (6)
I believe the answer is:
Here is my best explanation:
'eccentric' is the definition.
'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).
'y' put after 'screw' is 'SCREWY'.
'is' is the link.
(Other definitions for screwy that I've seen before include "Slightly mad", "like a prison officer?", "slightly mad", "weird", "Crazy", "Odd, eccentric", "Off one's rocker", "Daft (with a thread?)", "cuckoo?" and "Eccentric".)