Tree to be a problem afoot? (5)
I believe the answer is:
acorn
'tree to be' is the definition.
(I've seen this in another clue)
'a problem afoot?' is the wordplay.
'problem afoot?' becomes 'corn' (I can't justify this - if you can you should believe this answer much more).
'a'+'corn'='ACORN'
Can you help me to learn more?
(Other definitions for acorn that I've seen before include "Potential oak" , "Potentially future tree" , "From which a tall tree may grow" , "Tree seed" , "Model in stone" .)