Rule concerning mixing gin (5)
I believe the answer is:
reign
'rule' is the definition.
(synonyms)
'concerning mixing gin' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'mixing' indicates an anagram.
'gin' is an anagram of 'ign'.
're'+'ign'='REIGN'
(Other definitions for reign that I've seen before include "Have sovereign power" , "Govern" , "Wield the sceptre" , "Period on the throne" , "Period of a monarchy" .)