Completely knock barnacle off bar (5)
I believe the answer is:
clean
'completely' is the definition.
(thesaurus)
'knock barnacle off bar' is the wordplay.
'knock' indicates anagramming the letters.
'off' indicates named letters should be taken away ('off' can mean to remove).
'barnacle' with 'bar' taken away is 'nacle'.
'nacle' is an anagram of 'CLEAN'.
(Other definitions for clean that I've seen before include "Untainted" , "Complete" , "remove filth" , "Without stain" , "Devoid of dirt" .)