Part of crust contains special fibre (5)
I believe the answer is:
sisal
'fibre' is the definition.
(sisal is a kind of fibre)
'part of crust contains special' is the wordplay.
'part of' means one lot of letters goes inside another.
'crust' becomes 'sial' (I have seen 'Light crust' mean 'sial' so perhaps 'crust' could also mean 'sial').
'contains special' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'sial' placed around 's' is 'SISAL'.
Can you help me to learn more?
(Other definitions for sisal that I've seen before include "Fibre from agave plant" , "Type of fibre" , "Fibrous plant used for rope" , "Plant fibre used for making rope" , "Hemp fibre" .)