A good child on endless trail that's seen in cave (10)
I believe the answer is:
'that's seen in cave' is the definition.
(stone formation found in caves)
'a good child on' is the wordplay.
'a good' becomes 'stalag' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'child' becomes 'mite' (I've seen this before).
'on' says to put letters next to each other.
'endless trail' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for stalagmite that I've seen before include "Pillar of the underground" , "One that grows up" , "Cave floor growth" , "A mound of calcium carbonate" , "What may form on cave floor" .)