Protein in wheat initially needed to support muscle (6)
I believe the answer is:
gluten
'protein in wheat' is the definition.
'initially needed to support muscle' is the wordplay.
'initially' suggests taking the first letters.
'to support' means one lot of letters go next to another (in a down clue, some letters hold others up).
'muscle' becomes 'glute' (short for gluteus).
The initial letter of 'needed' is 'n'.
'n' put after 'glute' is 'GLUTEN'.
(Other definitions for gluten that I've seen before include "Protein found in cereals" , "Sticky protein present in cereal grains" , "Gluey protein substance in wheat" , "Substance in cereal grain" , "Protein present in wheat" .)