Service provided by leading store (6)
I believe the answer is:
upkeep
'service' is the definition.
'by leading store' is the wordplay.
'by' says to put letters next to each other.
'leading' becomes 'up' ('up' can be similar in meaning to 'leading').
'store' becomes 'keep'.
'up' next to 'keep' is 'UPKEEP'.
'provided' acts as a link.
(Other definitions for upkeep that I've seen before include "Cost of support" , "Maintenance costs" , "Cost of maintenance, say household" , "Looking after" , "Running costs" .)