Means to go west on large boat (4)
I believe the answer is:
yawl
'boat' is the definition.
(I've seen this before)
'means to go west on large' is the wordplay.
'means' becomes 'way' ('way' can be a synonym of 'means'**).
'to go west' says the letters should be written in reverse.
'on' says to put letters next to each other.
'large' becomes 'l'.
'way' in reverse letter order is 'yaw'.
'yaw'+'l'='YAWL'
(Other definitions for yawl that I've seen before include "boat? The reverse" , "Two-masted sailing boat" , "A two-master" , "Sailing-ship" , "Ships small boat" .)