Tiber diverted through channel? That's immoral (9)
I believe the answer is:
libertine
'immoral' is the definition.
(similar in meaning)
'tiber diverted through channel?' is the wordplay.
'diverted' indicates an anagram (letters must be diverted to different positions).
'through' means one lot of letters goes inside another (inserted letters go through the word).
'channel?' becomes 'line' (line is a kind of channel).
'tiber' anagrammed gives 'ibert'.
'ibert' inserted into 'line' is 'LIBERTINE'.
'that's' is the link.
(Other definitions for libertine that I've seen before include "Sensually indulgent person" , "Don Giovanni perhaps" , "Say, Don Giovanni" , "Person who live immorally - sounds free" , "Debauchee" .)