A direction to lower a boat (4)
I believe the answer is:
scow
'a boat' is the definition.
(scow is a kind of boat)
'a direction to lower' is the wordplay.
'a direction' becomes 's' (S is an example).
'to lower' becomes 'cow' (**).
's'+'cow'='SCOW'
(Other definitions for scow that I've seen before include "Low boat" , "ighter" , "Vessel" , "Maybe lighter" , "Flat-bottomed boat for cows" .)