Grain store accommodates old nobleman (5)
I believe the answer is:
baron
'nobleman' is the definition.
(baron is a kind of nobleman)
'grain store accommodates old' is the wordplay.
'grain store' becomes 'barn' (I am not sure about this - if you are sure you should believe this answer much more).
'accommodates' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'barn' enclosing 'o' is 'BARON'.
Can you help me to learn more?
(Other definitions for baron that I've seen before include "Noble rank" , "A lord; sounds like childless" , "Lowest rank of British nobility" , "Titled man for a portion of beef" , "powerful man owning lots of dailies?" .)