Free to wander sandbar (4)
I believe the answer is:
reef
'sandbar' is the definition.
(thesaurus)
'free to wander' is the wordplay.
'to wander' indicates an anagram.
'free' with letters rearranged gives 'REEF'.
(Other definitions for reef that I've seen before include "Submerged barrier" , "Bank of jagged rock near the surface of the sea" , "Gather up" , "Marine formation" , "one gathered on yacht" .)