Untie a reef knot (4)
I believe the answer is:
free
'untie' is the definition.
(I've seen this before)
'a reef knot' is the wordplay.
'knot' indicates anagramming the letters (I've seen 'knotted' mean this).
'reef' anagrammed gives 'FREE'.
(Other definitions for free that I've seen before include "Untrammelled" , "Unrestrained" , "As cheap as can be!" , "Pro bono" , "Unfettered" .)