Is to set off round about six - flying (6)
I believe the answer is:
exists
'is' is the definition.
The definition and answer can be both to do with fundamental existence as well as being verbs in their -s form.
Maybe they are linked in a way I don't understand?
'set off round about six flying' is the wordplay.
'set off round' becomes 'ets' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' is an insertion indicator.
'flying' is an anagram indicator (letters fly around into new positions).
'six' with letters rearranged gives 'xis'.
'ets' going around 'xis' is 'EXISTS'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for exists that I've seen before include "Subsists" , "Has being, is actual" , "Is present in the universe" , "Has objective reality" , "Has life" .)