Mature swan on river island (5)
I believe the answer is:
ripen
'mature' is the definition.
(ripening is a kind of maturing)
'swan on river island' is the wordplay.
'swan' becomes 'pen' (pen is a kind of swan).
'on' means one lot of letters go next to another.
'river' becomes 'r'.
'island' becomes 'i'.
'r'+'i'='ri'
'pen' put after 'ri' is 'RIPEN'.
(Other definitions for ripen that I've seen before include "Grow to maturity like fruit" , "come near to falling" , "Come to fruition" , "Become ready to eat" , "Become ready to pick" .)