Crude, when by inference? (5-4)

'inference?' is the definition.
The answer and definition are not the same part of speech.

'crude when' is the wordplay.
I cannot quite see how this works, but
'crude' could be 'rough' (similar in meaning) and 'rough' is present in the answer.
an anagram of 'when' is 'hewn' which is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

'by' acts as a link.

Can you help me to learn more?