If you lose this, you won't know what's going on (6)
I believe the answer is:
thread
'if you lose this you won't know what's going' is the definition.
The definition suggests a singular noun which matches the answer.
'on' is the wordplay.
I cannot really see how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for thread that I've seen before include "Subject" , "Fine cord of twisted fibers" , "Dearth of stitching material" , "Strand for sewing" , "Ridge around a screw" .)