Utter endless heresy, wildly (5)
I believe the answer is:
sheer
'utter' is the definition.
(synonyms)
'endless heresy wildly' is the wordplay.
'endless' means to remove the last letter.
'wildly' indicates anagramming the letters.
'heresy' with its final letter removed is 'heres'.
'heres' anagrammed gives 'SHEER'.
(Other definitions for sheer that I've seen before include "Very thin; unmitigated" , "Absolutely - fine - straight up" , "Utter - precipitous" , "Complete" , "sudden change of direction" .)