Rascal went back on a promise about boiling tea (10)
I believe the answer is:
degenerate
'rascal' is the definition.
'went back on a promise about boiling tea' is the wordplay.
'went back on a promise' becomes 'reneged'.
'about' shows that the letters should be reversed in order.
'boiling' is an anagram indicator.
'reneged' back-to-front is 'degener'.
'tea' with letters rearranged gives 'ate'.
'degener'+'ate'='DEGENERATE'
(Other definitions for degenerate that I've seen before include "sink" , "dishonourable" , "Rake" , "Decline physically or morally" , "Debased, degraded" .)