Smooth as keels can be (5)
I believe the answer is:
sleek
'smooth' is the definition.
(sleek means smooth or glossy)
'keels can be' is the wordplay.
'can be' indicates anagramming the letters.
'keels' is an anagram of 'SLEEK'.
'as' is the link.
(Other definitions for sleek that I've seen before include "Well groomed" , "Pleasingly smooth" , "Aerodynamic, streamlined" , "Looking prosperous" , "Excessively well-groomed" .)