Most indecent cleavage is in Paris (6)
I believe the answer is:
ripest
'most indecent' is the definition.
Both the answer and definition are superlative adjectives. Perhaps there's a link between them I don't understand?
'cleavage is in paris' is the wordplay.
'cleavage' becomes 'rip' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'is in paris' becomes 'est' ('is' in French).
'rip'+'est'='RIPEST'
Can you help me to learn more?
(Other definitions for ripest that I've seen before include "to be furthest from green" , "Most mature and ready to eat" , "Most ready for harvesting or eating" .)