It's costly to keep swans (four) in the river (9)
I believe the answer is:
expensive
'it's costly' is the definition.
(similar in meaning)
'to keep swans four in the river' is the wordplay.
'to keep' indicates putting letters inside.
'swans' becomes 'pens' (pen is a kind of swan).
'four' becomes 'IV' (Roman numerals).
'in the' means one lot of letters goes inside another.
'river' becomes 'exe' (River Exe).
'iv' put within 'exe' is 'exive'.
'pens' put into 'exive' is 'EXPENSIVE'.
(Other definitions for expensive that I've seen before include "Highly-priced" , "Very costly or dear" , "priceless" , "not going for a song" , "High in price" .)