Fly low with nobody on board (8)
I believe the answer is:
mosquito
'fly' is the definition.
(I've seen this before)
'low with nobody on board' is the wordplay.
'low' becomes 'moo' (both can mean to make a cow noise).
'with' is an insertion indicator.
'nobody on board' becomes 'squit' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'moo' going around 'squit' is 'MOSQUITO'.
Can you help me to learn more?
(Other definitions for mosquito that I've seen before include "Disease-carrying insect" , "Insect, the female of which sucks blood" , "Plane used in WWII" , "Disease-bearing fly" , "Blood-sucker" .)