Free to wander sandbar (4)
I believe the answer is:
reef
'sandbar' is the definition.
(thesaurus)
'free to wander' is the wordplay.
'to wander' is an anagram indicator.
'free' is an anagram of '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" .)