Means to reverse large sailing vessel (4)
I believe the answer is:
yawl
'sailing vessel' is the definition.
(I know that yawl is a type of sailing vessel)
'means to reverse large' is the wordplay.
'means' becomes 'way' (a way or means of doing something).
'to reverse' says the letters should be written in reverse.
'large' becomes 'L' (eg in clothes sizes).
'way' reversed gives 'yaw'.
'yaw'+'l'='YAWL'
(Other definitions for yawl that I've seen before include "Two-masted sailing boat" , "boat? The reverse" , "Ships small boat" , "Sailing vessel with two masts" , "A two-master" .)