Helmsman to steer south-west, carried by third person (8)
I believe the answer is:
coxswain
'helmsman' is the definition.
(coxswain is a kind of helmsman)
'steer south-west carried by third person' is the wordplay.
'steer' becomes 'ox'.
'south west' becomes 'sw'.
'carried by' is an insertion indicator.
'third person' becomes 'cain' (biblical third person after Adam and Eve).
'ox'+'sw'='oxsw'
'oxsw' placed inside 'cain' is 'COXSWAIN'.
'to' is the link.
(Other definitions for coxswain that I've seen before include "Person steering a boat" , "Rowing boat pilot" , "one steers" , "Person at the helm of a racing crew" , "Lifeboat helmsman" .)