River in a bad way — just a brook (4)
I believe the answer is:
rill
'a brook' is the definition.
(I've seen this in another clue)
'river in a bad way' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'in a bad way' becomes 'ill' ('in a bad way' can mean seriously ill or injured).
'r'+'ill'='RILL'
'just' acts as a link.
(Other definitions for rill that I've seen before include "Flow of water" , "Burn" , "Little stream" , "Small brook" , "Brooklet" .)