Pervert already sat in a perverted manner (4,6)
I believe the answer is:
lead astray
'pervert' is the definition.
(I've seen this before)
'already sat in a perverted manner' is the wordplay.
'in a perverted manner' indicates an anagram (a corrupted or perverted spelling).
'already'+'sat'='alreadysat'
'alreadysat' is an anagram of 'LEAD ASTRAY'.
(Other definitions for lead astray that I've seen before include "Seduce into wrongdoing" , "Encourage departure from the straight and narrow?" , "Misdirect - seduce" , "Draw into wrong course" , "Pervert" .)