Unfasten a reef knot? (4)
I believe the answer is:
free
'unfasten' is the definition.
(thesaurus)
'a reef knot?' is the wordplay.
'knot?' indicates anagramming the letters (I've seen 'knotted' mean this).
'reef' with letters rearranged gives 'FREE'.
(Other definitions for free that I've seen before include "Rescue" , "Not fixed; not in use" , "Unrestrained" , "Available - for love" , "unemployed" .)