A heavy arrow mostly is hard to stop (7)
I believe the answer is:
abolish
'stop' is the definition.
(to abolish is to stop or ban)
'a heavy arrow mostly is hard' is the wordplay.
'heavy arrow' becomes 'bolt' (an arrow from a crossbow).
'mostly' means to remove the last letter.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'bolt' with its last letter taken off is 'bol'.
'a'+'bol'+'is'+'h'='ABOLISH'
'to' is the link.
(Other definitions for abolish that I've seen before include "Put an end to, cancel" , "Get rid of" , "Put an end to, formally" , "Annul - expunge" , "Cancel or abrogate" .)