The most common and most likely answer to this crossword clue is the 6 letter word SLUICE
I've seen this crossword clue in The Guardian
S
L
U
I
C
E
More clues leading to the result SLUICE
Buddy explains!

The straight part
We know that "Watergate" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case watergate = sluice.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 SLUICE mean?
SLUICE noun- An artificial passage for water, fitted with a valve or gate, for example in a canal lock or a mill stream, for stopping or regulating the flow.
- A water gate or floodgate.
- Hence, an opening or channel through which anything flows; a source of supply.
- The stream flowing through a floodgate.
- A long box or trough through which water flows, used for washing auriferous earth.
- An instance of wh-stranding ellipsis, or sluicing.
- To emit by, or as by, flood gates.
- To wet copiously, as by opening a sluice
- To wash with, or in, a stream of water running through a sluice.
- (more generally) To wash (down or out).
- To flow, pour.
- To elide the C` in a coordinated wh-question. See sluicing.