Block up hideout, heading off to provide protein (6)
I believe the answer is:
gluten
'protein' is the definition.
(gluten is a kind of protein)
'block up hideout heading off' is the wordplay.
'block up' becomes 'glut' (I can't justify this - if you can you should believe this answer much more).
'hideout' becomes 'den' (synonyms).
'heading off' means to remove the first letter.
'den' with its first letter taken away is 'en'.
'glut'+'en'='GLUTEN'
'to provide' acts as a link.
Can you help me to learn more?
(Other definitions for gluten that I've seen before include "Part of wheat" , "Protein found in cereals" , "Protein in grain that gives cohesiveness to dough" , "Protein left after starch is removed from cereal grains" , "Substance in cereals" .)