Cave in Split? Slip up climbing inside it (9)
I believe the answer is:
surrender
'cave in split?' is the definition.
'surrender' can be an answer for 'cave' (thesaurus). I'm unsure of the 'in split?' bit.
'slip up climbing inside it' is the wordplay.
'slip up' becomes 'err' (to err is to make an error).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'inside' is an insertion indicator.
'it' becomes 'sunder' (I can't explain this - if you can you should give a lot more credence to this answer).
'err' backwards is 'rre'.
'rre' going inside 'sunder' is 'SURRENDER'.
Can you help me to learn more?
(Other definitions for surrender that I've seen before include "Resign" , "Yield - release" , "Yield to the control of another" , "Cease resistance" , "Capitulation" .)