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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'hideout' becomes 'den' (synonyms).
'heading off' means to remove the first letter.
'den' with its initial letter taken away is 'en'.
'glut'+'en'='GLUTEN'
'to provide' is the 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" .)