The most common and most likely answer to this crossword clue is the 5 letter word SUMPS
I've seen this crossword clue in Independent
S
U
M
P
S
More clues leading to the result SUMPS
Buddy explains!

The straight part
We know that "reservoirs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reservoirs = sumps.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SUMPS mean?
SUMPS noun- A hollow or pit into which liquid drains, such as a cesspool, cesspit or sink.
- The lowest part of a mineshaft into which water drains.
- A completely flooded cave passage, sometimes passable by diving.
- The crankcase or oil reservoir of an internal combustion engine.
- The pit at the lowest point in a circulating or drainage system (FM 55-501).
- An intentional depression around a drain or scupper that promotes drainage.
- Of a cave passage, to end in a sump, or to fill completely with water on occasion.