It'll become a pleasant cadence (4)
I believe the answer is:
lilt
'a pleasant cadence' is the definition.
(I've seen this in another clue)
'it'll become' is the wordplay.
'become' indicates anagramming the letters.
'itll' is an anagram of 'LILT'.
(Other definitions for lilt that I've seen before include "Cheerful tune" , "Jaunty musical rhythm" , "Light graceful swinging rhythm" , "Pleasant gentle accent" , "Rise and fall" .)