Organism that's disturbingly rife under rubbish (7)
I believe the answer is:
rotifer
'organism' is the definition.
(rotifer is a kind of organism)
'disturbingly rife under rubbish' is the wordplay.
'disturbingly' is an anagram indicator (I've seen this in other clues).
'under' means one lot of letters go next to another.
'rubbish' becomes 'rot' (I've seen this before).
'rife' is an anagram of 'ifer'.
'ifer' put after 'rot' is 'ROTIFER'.
'that's' acts as a link.
(Other definitions for rotifer that I've seen before include "Aquatic creature" , "Tiny animal" , "Simple animal" .)