Eating badly, first of lentils consumed for protein (7)
I believe the answer is:
gelatin
'protein' is the definition.
(gelatin is a kind of protein)
'eating badly first of lentils consumed' is the wordplay.
'badly' indicates an anagram (letters in the wrong order).
'first of lentils' becomes 'l' (1st letter of 'lentils').
'consumed' is an insertion indicator.
'eating' is an anagram of 'geatin'.
'geatin' placed around 'l' is 'GELATIN'.
'for' acts as a link.
(Other definitions for gelatin that I've seen before include "Thickener" , "Jelling agent" , "Tag line (anag.); glue" , "Glue made from animal extracts" , "Colourless protein extracted from animal bones" .)