Stinking mud has risen on the contrary (6)
I believe the answer is:
putrid
'stinking' is the definition.
(synonyms)
'mud has risen on the contrary' is the wordplay.
'mud' becomes 'dirt'.
'has' says to put letters next to each other.
'risen' becomes 'up' (similar in meaning).
'on the contrary' shows that the letters should be reversed in order (letters in the opposite or contrary direction).
'dirt'+'up'='dirtup'
'dirtup' back-to-front is 'PUTRID'.
(Other definitions for putrid that I've seen before include "Bad smelling, rotting" , "contaminated" , "Rancid" , "Decayed, rotten" , "Turned" .)