Spore-dense structures die (5)
I believe the answer is:
mould
This clue might be a double definition.
'spore' is the first definition.
(thesaurus)
'dense structures die' is the second definition.
'mould' can be an answer for 'die' (thesaurus). I am not certain of the 'dense structures' bit.
Can you help me to learn more?
(Other definitions for mould that I've seen before include "Cause of discoloration" , "Shape" , "Guide or control the development of" , "Penicillin was discovered in this" , "Must" .)