Cost of maintaining high stock (6)
I believe the answer is:
upkeep
'cost of maintaining' is the definition.
(I've seen this before)
'high stock' is the wordplay.
'high' becomes 'up' ('up' is associated in meaning with 'high').
'stock' becomes 'keep' (I've seen this before**).
'up'+'keep'='UPKEEP'
(Other definitions for upkeep that I've seen before include "Cost of support" , "Looking after" , "Cost of maintenance, say household" , "payments?" , "Maintenance costs" .)