Huge effort required to admit old prejudice (7)
I believe the answer is:
bigotry
'prejudice' is the definition.
(I've seen this before)
'huge effort required to admit old' is the wordplay.
I cannot quite understand how this works, but
'huge' could be 'big' ('big' can be similar in meaning to 'huge') and 'big' is found within the answer.
'effort' could be 'try' ('try' can be a synonym of 'effort') and 'try' is found in the answer.
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found within the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bigotry that I've seen before include "Intolerance, prejudice on grounds of religion" , "Intolerant prejudice" , "Stubborn prejudice" , "Blind zeal" .)