Phone system Bury firm marketed at first (8)
I believe the answer is:
intercom
'phone system' is the definition.
(communication device)
'bury firm marketed at first' is the wordplay.
'bury' becomes 'inter' (to inter is to put into the ground).
'firm' becomes 'co' (a firm is a company).
'at first' indicates taking the first letters.
The first letter of 'marketed' is 'm'.
'inter'+'co'+'m'='INTERCOM'
(Other definitions for intercom that I've seen before include "way to communicate" , "Short-range telephone system" , "Phone system inside building" , "Speaker system" , "short range phone" .)