Quick! It's nearly all run away! (5)
I believe the answer is:
fleet
'quick' is the definition.
(I've seen this in another clue)
'nearly all run away' is the wordplay.
'nearly all' means to remove the last letter (most but not all of the word).
'run' becomes 'flee' (to flee is to run away).
'away' becomes 'et' (I can't justify this - if you can you should believe this answer much more).
'flee' with its final letter taken away is 'fle'.
'fle'+'et'='FLEET'
'it's' is the link.
Can you help me to learn more?
(Other definitions for fleet that I've seen before include "good turn of speed" , "Fast - naval force" , "Group of planes, ships or cars" , "bowler famously intercepted one once" , "cars en masse?" .)