Watergate clue is to be solved (6)
I believe the answer is:
sluice
'watergate' is the definition.
(I've seen this before)
'clue is to be solved' is the wordplay.
'to be solved' indicates anagramming the letters.
'clue'+'is'='clueis'
'clueis' anagrammed gives 'SLUICE'.
(Other definitions for sluice that I've seen before include "Wash or rinse with a stream of water" , "Water drainage channel" , "Sliding gate for controlling flow of water in stream" , "Artificial channel" , "Water flow control gate" .)