Bug pervades litter and adds oxygen (9)
I believe the answer is:
eavesdrop
'bug' is the definition.
(I know this)
'pervades litter and adds oxygen' is the wordplay.
I cannot really see how this works, but
'oxygen' could be 'o' (atomic symbol for oxygen) and 'o' is found within the answer.
an anagram of 'pervades' is 'eavesdrp' which is within the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for eavesdrop that I've seen before include "Hold secret hearing" , "spy" , "Deprave so to listen secretly" , "Overhear" , "Listen to bug" .)