Spread of a mould, starting in alimentary canal (5)
I believe the answer is:
gamut
'spread' is the definition.
(thesaurus)
'a mould starting in alimentary canal' is the wordplay.
'starting' indicates taking the first letters.
'in' is an insertion indicator.
'alimentary canal' becomes 'gut' (I've seen this before).
The first letter of 'mould' is 'm'.
'a'+'m'='am'
'am' going inside 'gut' is 'GAMUT'.
'of' is the link.
(Other definitions for gamut that I've seen before include "Entire scale" , "Complete range or scale" , "Full extent" , "Complete extent or range" , "Complete range (of something)" .)