Rat chewed part way through Hobnob producing mould (6)
I believe the answer is:
matrix
'mould' is the definition.
(matrix is a kind of mould)
'rat chewed part way through hobnob' is the wordplay.
'chewed' is an anagram indicator.
'part way through' means one lot of letters goes inside another.
'hobnob' becomes 'mix' (I am not sure about this - if you are sure you should believe this answer much more).
'rat' anagrammed gives 'atr'.
'atr' going within 'mix' is 'MATRIX'.
'producing' is the link.
Can you help me to learn more?
(Other definitions for matrix that I've seen before include "Arrangement of numbers" , "Grid-like pattern of elements" , "Mathematical array of numbers" , "Array of numbers in rows and columns" , "Grid-like array of rat mix" .)