Give warning as climbing up red ash carelessly splits bark (10)

I believe the answer is:
foreshadow
'give warning' is the definition.
(I've seen this before)
'climbing up red ash carelessly splits bark' is the wordplay.
I cannot really understand how this works, but
'red' could be 'r' (I've seen this in other clues) and 'r' is located in the answer.
an anagram of 'ash' is 'sha' which is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'as' acts as a link.
Can you help me to learn more?
(Other definitions for foreshadow that I've seen before include "Give warning" , "Warn" , "Presage" .)
