Happening immediately, some run-ins tantalise (7)
I believe the answer is:
instant
'happening immediately' is the definition.
(I've seen this before)
'ins tantalise' is the wordplay.
'tantalise' becomes 'tant' (I can't explain this - if you can you should give a lot more credence to this answer).
'ins'+'tant'='INSTANT'
'some run' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for instant that I've seen before include "Jiffy" , "Sudden; moment" , "Fast, flash" , "Happening immediately" , "Split second" .)