North Leeds transformed by Stallone forever (9)
I believe the answer is:
endlessly
'forever' is the definition.
(I've seen this in another clue)
'north leeds transformed by stallone' is the wordplay.
'north' becomes 'N' (abbreviation).
'transformed' is an anagram indicator.
'by' is a charade indicator (letters next to each other).
'stallone' becomes 'sly' (nickname for Sylvester Stallone).
'n'+'leeds'='nleeds'
'nleeds' is an anagram of 'endles'.
'endles'+'sly'='ENDLESSLY'
(Other definitions for endlessly that I've seen before include "Limitlessly" , "Ad infinitum" , "all the time" , "in perpetuity" , "Ceaselessly" .)