Cutting cover of tree mostly over border (6)
I believe the answer is:
barbed
'cutting cover' is the definition.
'barbed' can be an answer for 'cutting' (I've seen this in another clue). I am not certain of the 'cover' bit.
'of tree mostly over border' is the wordplay.
'of tree' becomes 'ba' (I can't explain this - if you can you should give a lot more credence to this answer).
'mostly' means to remove the last letter (most but not all of the word).
'over' becomes 're'.
'border' becomes 'bed' (a flower bed is the border of a garden).
're' with its final letter taken off is 'r'.
'ba'+'r'+'bed'='BARBED'
Can you help me to learn more?
(Other definitions for barbed that I've seen before include "Wounding, spiteful" , "Like cruel joke" , "Of remark, sharp and wounding" , "deliberately hurtful?" , "(Of wire) having short spikes at intervals" .)