Toper’s second return, always half-ignored and not to be mentioned (3,6)
I believe the answer is:
top secret
'return always half-ignored and not to be mentioned' is the definition.
I can't tell whether this definition defines the answer.
'toper's second' is the wordplay.
I cannot quite understand how this works, but
'second' could be 'sec' (synonyms) and 'sec' is located in the answer.
an anagram of 'toper' is 'topre' which is present in the remaining letters.
A single letter 't' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for top secret that I've seen before include "Like highly guarded information" , "As 007 mission" , "Well-hidden" , "(Of information) strictly not for public consumption" , "restricted" .)