Unacceptable extremes, not to be gone into (3,6)
I believe the answer is:
off limits
'gone into' is the definition.
The answer and definition are different parts of speech. However, adjectives and past participle verbs occasionally define each other.
'unacceptable extremes not' is the wordplay.
'unacceptable' becomes 'off' (I've seen this in other clues).
'extremes not' becomes 'limits' (I can't explain this - if you can you should give a lot more credence to this answer).
'off'+'limits'='OFF-LIMITS'
'to be' acts as a link.
Can you help me to learn more?
(Other definitions for off limits that I've seen before include "Out of bounds" , "not to be visited" , "Out-of-bounds" , "beyond the pale" , "Where boundaries must have been crossed" .)