Merchant ship carrying boulders periodically (10)
I believe the answer is:
wholesaler
'merchant' is the definition.
(I've seen this before)
'ship carrying boulders periodically' is the wordplay.
'ship' becomes 'whaler' (whaler is a kind of ship).
'carrying' means one lot of letters goes inside another.
'periodically' indicates alternate letters (letters must be selected 'every so often').
The alternate letters of 'boulders' are 'oles'.
'whaler' going around 'oles' is 'WHOLESALER'.
(Other definitions for wholesaler that I've seen before include "Merchant" , "firm that supplies in bulk" , "Retailer's supplier" , "retailer of large quantities" , "Bulk trader" .)