Publican's secret stock inside (9)
I believe the answer is:
innkeeper
'publican's' is the definition.
(a publican runs a pub)
'secret stock inside' is the wordplay.
'secret' becomes 'inner' (I've seen this before).
'stock' becomes 'keep' (I've seen this before).
'inside' means one lot of letters goes inside another.
'inner' placed around 'keep' is 'INNKEEPER'.
(Other definitions for innkeeper that I've seen before include "For example, Mistress Quickly" , "Job" , "Accommodating type" , "Pub licensee" , "Mine host" .)