Rain only half stopped -- outside river swelled (9)
I believe the answer is:
increased
'swelled' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I don't see?
'rain only half stopped outside river' is the wordplay.
'only half' means to take half the letters.
'stopped' becomes 'ceased' ('cease' can be a synonym of 'stop').
'outside' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'rain' halved is 'in'.
'ceased' placed around 'r' is 'creased'.
'in'+'creased'='INCREASED'
Can you help me to learn more?
(Other definitions for increased that I've seen before include "Went up" , "Grew greater" , "Augmented" , "Made bigger or more" , "Made greater" .)