Mob down by 500 attending pub one used to open (7)
I believe the answer is:
crowbar
'one used to open' is the definition.
(tool for forcing things open)
'mob down by 500 attending pub' is the wordplay.
'mob' becomes 'crowd' (I've seen this before).
'down by' indicates named letters should be taken away (to be down something is to be lacking it).
'500' becomes 'D' (Roman numeral for 500).
'attending pub' becomes 'bar' (both are drinking establishments. I am not sure about the 'attending' bit.).
'crowd' with 'd' removed is 'crow'.
'crow'+'bar'='CROWBAR'
(Other definitions for crowbar that I've seen before include "Purchase may be obtained from this" , "Metal rod used as a lever" , "Levering tool" , "Burglar's tool, perhaps" , "In America, one prizes" .)