Second person steering fish ahead of stream (2-6)
I believe the answer is:
co-driver
'second person steering' is the definition.
'fish ahead of stream' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish).
'ahead of' means one lot of letters go next to another (I've seen this in other clues).
'stream' becomes 'river' (river is a kind of stream**).
'cod'+'river'='CO-DRIVER'
(Other definitions for co-driver that I've seen before include "Rally partner" , "Rallying partner" , "Divorcer (anag.)" .)