All quiet in the reservoir (4)
I believe the answer is:
sump
'the reservoir' is the definition.
(sump is a kind of reservoir)
'all quiet' is the wordplay.
'all' becomes 'sum' (I've seen this before).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'sum'+'p'='SUMP'
'in' is the link.
(Other definitions for sump that I've seen before include "Hole collecting water for pump" , "Pit collecting water/oil" , "Water collector" , "Pit receiving drainage" , "lowest point of mine" .)