Measure by sexism? An odd activity! (4,4,5)
I believe the answer is:
body mass index
'measure' is the definition.
(measure of being overweight or underweight)
'by sexism? an odd activity' is the wordplay.
'activity' indicates anagramming the letters (letters become active).
'by'+'sexism'+'an'+'odd'='bysexismanodd'
'bysexismanodd' with letters rearranged gives 'BODY MASS INDEX'.
(Other definitions for body mass index that I've seen before include "rating figure?" , "A measure of fatness" , "Measure of obesity" , "Measure of someone's weight in relation to height" , "Measure of fat/muscle ratio" .)