River containing single example of vermin - that’s clear (9)
I believe the answer is:
exonerate
'clear' is the definition.
(synonyms)
'river containing single example of vermin' is the wordplay.
'river' becomes 'exe' (River Exe).
'containing' is an insertion indicator.
'single' becomes 'one' (similar in meaning).
'example of vermin' becomes 'rat'.
'one'+'rat'='onerat'
'exe' placed around 'onerat' is 'EXONERATE'.
'that's' is the link.
(Other definitions for exonerate that I've seen before include "Excuse" , "Free from obligation, blame" , "Justify" , "Acquit" , "free of charge" .)