Engineer on metric telephone system (8)
I believe the answer is:
intercom
'telephone system' is the definition.
(I've seen this before)
'engineer on metric' is the wordplay.
'engineer' indicates an anagram.
'on'+'metric'='onmetric'
'onmetric' with letters rearranged gives 'INTERCOM'.
(Other definitions for intercom that I've seen before include "Receiver" , "Device for two-way talking" , "Two-way speaking device" , "way to communicate" , "short range phone" .)