Measure of keenness to offer unflattering comments about big name (7)
I believe the answer is:
mustard
'measure of keenness' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'unflattering comments about big name' is the wordplay.
'unflattering comments' becomes 'mud' (I have seen 'Adverse comments' mean 'mud' so perhaps 'comments' could also mean 'mud'. I am not sure about the 'unflattering' bit.).
'about' indicates putting letters inside.
'big name' becomes 'star' (I've seen this before).
'mud' going around 'star' is 'MUSTARD'.
'to offer' acts as a link.
Can you help me to learn more?
(Other definitions for mustard that I've seen before include "Pungent condiment" , "See 2" , "Hot stuff!" , "Murderer" , "Hot-tasting seed" .)