Work colleague conclusively classifying short fibrous protein (7)
I believe the answer is:
keratin
'fibrous protein' is the definition.
(I've seen this in another clue)
'work colleague conclusively classifying short' is the wordplay.
'conclusively' indicates one should take the final letters (the letter which concludes the word).
'classifying' becomes 'rating'.
'short' means to remove the last letter.
The last letters of 'work colleague' are 'ke'.
'rating' with its final letter taken off is 'ratin'.
'ke'+'ratin'='KERATIN'
(Other definitions for keratin that I've seen before include "Rate ink (anag) - fibrous protein" , "Hair, nails substance" , "Protein found in hair and nails" , "Constituent of hair/nails" , "Material in hooves and nails" .)