A duck and swan over fluid (4-5)
I believe the answer is:
open-ended
'fluid' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
'a duck and swan over' is the wordplay.
'a duck' becomes 'o' (resembles zero - 'duck' in cricket).
'and' says to put letters next to each other.
'swan' becomes 'pen' (pen is a kind of swan).
'over' becomes 'ended' (over can mean finished or ended).
'o'+'pen'+'ended'='OPEN-ENDED'
Can you help me to learn more?
(Other definitions for open-ended that I've seen before include "Allowing free debate with no set time to finish." , "Having no fixed limit, perhaps a discussion" , "With no fixed limit, not circumscribed" , "Without fixed limits or restrictions" , "Having no predetermined limit" .)