Unknown on second team is eccentric (6)
I believe the answer is:
Here is my best explanation:
'eccentric' is the definition.
(I've seen this before)
'unknown on second team' is the subsidiary indication.
'unknown' becomes 'y' (algebra).
'on' is a charade indicator (letters next to each other).
'second' becomes 's' (common abbreviation).
'team' becomes 'crew' (crew is a kind of team**).
'y' 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".)