Belittle unusual tip: no good for code breaking (10)
I believe the answer is:
decrypting
'code breaking' is the definition.
The definition suggests a present participle verb which matches the answer.
'belittle unusual tip no good' is the wordplay.
'belittle' becomes 'decry' (I've seen this before**).
'unusual' indicates an anagram.
'no' becomes 'n' (common abbreviation).
'good' becomes 'g' (abbreviation).
'tip' with letters rearranged gives 'pti'.
'decry'+'pti'+'n'+'g'='DECRYPTING'
'for' acts as a link.
Can you help me to learn more?
(Another definition for decrypting that I've seen is " Cracking".)