Just as they're leaving, see that it's in progress? (7)
I believe the answer is:
ongoing
'it's in progress?' is the definition.
('ongoing' can be a synonym of 'in progress')
'just as they're leaving see' is the wordplay.
I cannot quite see how this works, but
'as' could be 'in' (I've seen this in other clues) and 'in' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'that' is the link.
Can you help me to learn more?
(Other definitions for ongoing that I've seen before include "Currently in progress" , "still in operation" , "Actually in progress" , "Still in progress" , "Continually moving forward" .)