One doesn't hear it when it's dropped (8)
I believe the answer is:
aspirate
'one doesn't hear it' is the definition.
I can't judge whether this definition defines the answer.
'when it's dropped' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'it's dropped' becomes 'pirate' (I can't explain this - if you can you should give a lot more credence to this answer).
'as'+'pirate'='ASPIRATE'
Can you help me to learn more?
(Other definitions for aspirate that I've seen before include "Pronounce with an h, like parasite" , "First appearance of Hook" , "Letter pronounced while breathing out" , "Higgins's initial" , "roughly speaking?" .)