Aggregate's put round opening of petrol reservoirs (5)
I believe the answer is:
Here is my best explanation:
'reservoirs' is the definition.
(sump is a kind of reservoir)
'aggregate's put round opening of petrol' is the subsidiary indication.
'aggregate's' becomes 'sums' ('sum' can be a synonym of 'aggregate').
'put round' is an insertion indicator.
'opening of' suggests taking the first letters.
The first letter of 'petrol' is 'p'.
'sums' enclosing 'p' is 'sumps'.
(Other definitions for sumps that I've seen before include "Oil reservoirs in engines" and "Reservoirs".)