Not having got set, is off the mark (7)
I believe the answer is:
running
'the mark' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe there's a link between them I don't understand?
'not having got set is off' is the wordplay.
I cannot quite understand how this works, but
'not' could be 'un' (I've seen this in other clues) and 'un' is found within the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for running that I've seen before include "In operation" , "No chance if you're out of it" , "Managing; flowing" , "Moving fast, on track perhaps" , "in action" .)