One who once took part as Gradgrind, for instance (7)
I believe the answer is:
exactor
'one who once took part as gradgrind for instance' is the definition.
The definition suggests a singular noun which matches the answer.
'one who once took part' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'once' could be 'ex' (prefix meaning former or onetime) and 'ex' is found within the answer.
'part' could be 'act' (I have seen 'Part of play ' mean 'act' so perhaps 'part' could also mean 'act') and 'act' is found in the answer.
A single letter 'r' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for exactor that I've seen before include "He demands payment" , "Shylock" , "Demanding person" , "demanding sort" , "Blackmailer" .)