Rule, say, kept in cold storage (6)
I believe the answer is:
regime
'rule' is the definition.
(thesaurus)
'say kept in cold storage' is the wordplay.
'say' becomes 'eg'.
'kept in' indicates putting letters inside.
'cold storage' becomes 'rime' (I can't justify this - if you can you should give a lot more credence to this answer).
'eg' going into 'rime' is 'REGIME'.
Can you help me to learn more?
(Other definitions for regime that I've seen before include "Ruling system" , "Emigre (anag.)" , "Dominion of emigre" , "Plan for therapy or government in power" , "Routine" .)