Corrupt police officer oddly ratty (5)
I believe the answer is:
dirty
'corrupt' is the definition.
(similar in meaning)
'police officer oddly ratty' is the wordplay.
'police officer' becomes 'di' (detective inspector).
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'ratty' are 'rty'.
'di'+'rty'='DIRTY'
(Other definitions for dirty that I've seen before include "Muddy" , "Salacious" , "Not clean" , "Lewd" , "Begrimed" .)