Free duck and swan (4)
I believe the answer is:
open
'free' is the definition.
(similar in meaning)
'duck and swan' is the wordplay.
'duck' becomes 'o'.
'and' means one lot of letters go next to another.
'swan' becomes 'pen' (pen is a kind of swan).
'o'+'pen'='OPEN'
(Other definitions for open that I've seen before include "Unlock" , "unresolved" , "golf contest" , "Undo; exposed to attack" , "Without concealment" .)