Keeps inventory in locking equipment (6)
I believe the answer is:
stocks
'keeps' is the definition.
(I've seen this before)
'inventory in locking equipment' is the wordplay.
'inventory' becomes 'stock' (synonyms).
'in locking equipment' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'stock'+'s'='STOCKS'
Can you help me to learn more?
(Other definitions for stocks that I've seen before include "Amounts of goods kept by traders" , "In which miscreant is locked" , "Securities, investments" , "Old public punishment device" , "Plants; form of punishment" .)