Rung when away from company briefly by city chap (7)
I believe the answer is:
echelon
'rung' is the definition.
'away from company briefly by city chap' is the wordplay.
'away from company' becomes 'lone'.
'briefly' means to remove the last letter.
'by' means one lot of letters go next to another.
'city' becomes 'ec' (the City of London postcode).
'chap' becomes 'he'.
'lone' with its final letter taken away is 'lon'.
'ec'+'he'='eche'
'lon' put after 'eche' is 'ECHELON'.
'when' acts as a link.
(Other definitions for echelon that I've seen before include "Echo Len (anag.)" , "Level of command" , "Grade or level" , "Military stepped formation" , "Military formation; level of society" .)