Top grade hiding a sign of infamy (4,2,3,5)

'infamy' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'top grade hiding a sign' is the wordplay.
I cannot really see how this works, but
'grade' could be 'mark' (synonyms) and 'mark' is found in the answer.
'a' is present in the answer.
'top' could be 'best' (associated in meaning) and 'best' is present in the leftover letters.
'sign' could be 'he' (he is a kind of sign) and 'he' is located in the answer.
The remaining letters 'oft' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

'of' acts as a link.

Can you help me to learn more?