Hard border that could separate neighbours’ gardens (5)
I believe the answer is:
hedge
'hard border that could separate neighbours' gardens' is the definition.
'hedge' can be an answer for 'hard border'. I am unsure of the remainder of the definition.
'hard border' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'border' becomes 'edge' (synonyms).
'h'+'edge'='HEDGE'
Can you help me to learn more?
(Other definitions for hedge that I've seen before include "Avoid making definite statement" , "Try to avoid answering questions or dealing with issues" , "Bushy barrier" , "Field barrier" , "Avoid making a definite commitment" .)