Possible barrier to free movement? (4)
I believe the answer is:
reef
'possible barrier' is the definition.
'reef' can be an answer for 'barrier' (I have seen 'Submerged barrier' mean 'reef' so perhaps 'barrier' could also mean 'reef'). I am not certain of the 'possible' bit.
'free movement?' is the wordplay.
'movement?' is an anagram indicator (I've seen 'movement' mean this).
'free' anagrammed gives 'REEF'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for reef that I've seen before include "Rock near water's surface" , "Shorten sail" , "Rock formed of coral" , "Ridge of coral near the sea surface" , "Coral-ridge" .)