'Hard borders' is deliberately vague (6)
I believe the answer is:
hedges
'deliberately vague' is the definition.
I can't judge whether this definition defines the answer.
'hard borders' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'borders' becomes 'edges' (an edge is a border or margin).
'h'+'edges'='HEDGES'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for hedges that I've seen before include "Avoids answering" , "Buys or sells protective security" , "Equivocates" , "Boundaries of shrubs planted close together" , "Fences of bushes" .)