Terrible ham promises to change (13)
I believe the answer is:
metamorphosis
'change' is the definition.
(I've seen this before)
'terrible ham promises to' is the wordplay.
'terrible' indicates anagramming the letters.
'ham'+'promises'+'to'='hampromisesto'
'hampromisesto' is an anagram of 'METAMORPHOSIS'.
(Other definitions for metamorphosis that I've seen before include "Complete change of appearance" , "Total change in form" , "changing?" , "Process of transformation" , "Transmutation" .)