Porn supremo perhaps becoming less successful (7)
I believe the answer is:
sinking
'becoming less successful' is the definition.
(eg a business may sink or go under)
'porn supremo' is the wordplay.
'porn' becomes 'sin' (I can't explain this - if you can you should believe this answer much more).
'supremo' becomes 'king' (both can refer to a person with great authority).
'sin'+'king'='SINKING'
'perhaps' acts as a link.
(Other definitions for sinking that I've seen before include "Declining" , "Going down" , "failing" , "in decline" , "Falling to the bottom" .)