Chief plant-holder, overturned, almost cracked (3,6)
I believe the answer is:
top banana
'chief' is the definition.
(I've seen this in another clue)
'plant-holder overturned almost cracked' is the wordplay.
'plant holder' becomes 'pot' (I've seen this in another clue).
'overturned' says the letters should be written backwards.
'almost cracked' becomes 'banana' (I can't explain this - if you can you should give a lot more credence to this answer).
'pot' back-to-front is 'top'.
'top'+'banana'='TOP BANANA'
Can you help me to learn more?
(Other definitions for top banana that I've seen before include "figurehead?" , "1" , "Boss (slang)" , "leader" , "head" .)