To expel almost all of gang is hard (6)
I believe the answer is:
banish
'to expel' is the definition.
(banishing is a kind of expelling)
'almost all of gang is hard' is the wordplay.
'almost all of' means to remove the last letter.
'gang' becomes 'band' (I've seen this before).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'band' with its last letter taken away is 'ban'.
'ban'+'is'+'h'='BANISH'
(Other definitions for banish that I've seen before include "Expel from a community or group" , "Condemn to exile" , "'Cast out, ostracise (6)'" , "Cast out, ostracize" , "Expel or expatriate" .)