Proxy rule regarding unopened office (7)
I believe the answer is:
regency
'proxy rule' is the definition.
(rule by a regent on behalf of a monarch)
'regarding unopened office' is the wordplay.
'regarding' becomes 'Re'.
'unopened' means to remove the first letter.
'office' becomes 'agency' (synonyms).
'agency' with its initial letter taken away is 'gency'.
're'+'gency'='REGENCY'
(Other definitions for regency that I've seen before include "Years from 1811 to 1820 during George III's periods of insanity" , "Period of king's incapacity" , "British period, Eighteen-Eleven to Eighteen-Twenty" , "Interim ruling period on behalf of another" , "Old style" .)