Worried after pulse initially dropping 90% — getting air (9)
I believe the answer is:
ventilate
'air' is the definition.
(I know that air can be written as ventilate)
'worried after pulse initially' is the wordplay.
'worried' becomes 'ate' (something worrying may eat someone up).
'after' says to put letters next to each other.
'pulse initially' becomes 'ventil' (I can't justify this - if you can you should give a lot more credence to this answer).
'ate' put after 'ventil' is 'VENTILATE'.
'dropping 90% getting' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ventilate that I've seen before include "Let air enter and circulate" , "Expose to a current of fresh air" , "Fan" , "Superficial" , "Allow fresh air to circulate" .)