Fill a keel with some water (4)
I believe the answer is:
lake
'water' is the definition.
(lake is a kind of water)
'fill a keel with some' is the wordplay.
'some' indicates a hidden word.
'LAKE' is hidden within 'fill a keel with'.
(Other definitions for lake that I've seen before include "A pigment; a body of water" , "Large pond; pigment" , "Large area of water; pigment" , "Mere - organic pigment" , "E.g. Huron" .)