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 wordplay.
'aggregate's' becomes 'sums' ('sum' can be a synonym of 'aggregate').
'put round' means one lot of letters goes inside another.
'opening of' suggests taking the first letters.
The first letter of 'petrol' is 'p'.
'sums' going around 'p' is 'SUMPS'.
(Other definitions for sumps that I've seen before include "Oil reservoirs in car engines" and "Reservoirs".)