I have successfully exited Clapham Junction midway through a through ticket to break a journey.
I wanted to then touch in on Oyster to pop into London.
There didn’t seem to be a problem.
[...]
I've not had a problem breaking a journey at Clapham Junction, using paper tickets and asking the gateline staff.
However if at Clapham Junction you do just want to touch-in or touch-out using Oyster or contactless, there are standalone Oyster/contactless readers at both ends of platform 17 - both near the stairs at the bottom of the overbridge, and also near the stairs up from the subway. I
think there's also a reader at the bottom (subway end) of the stairs from platform 17.
There is also a couple of standalone Oyster/contactless readers that always seems to be switched on next to a normally closed overflow exit door (in the subway at the St John's Hill end, very close to the gateline. (However if there were RPIs nearby monitoring the gateline, using them might be the sort of thing that might attract the RPIs unneccesary attention - not because you are doing anything wrong, but understandably it could be interpreted as slightly suspicious.)
Lastly, it's not officially documented (well, I guess it is via FOI released documentation!), but it's currently possible to
start (i.e. touch-in) an Oyster/contactless journey by using a
pink reader (which are located on the Overground platforms, that's platforms 1 and 2) - but note you
cannot end (i.e. touch-out) an Oyster/contactless journey using a pink reader. (Several years ago it was possible to do both, but then TfL changed the logic - of course it's possible the logic could be changed again, so perhaps it's best not to rely on it!)
Perhaps easiest to just head to platform 17.