Simple problem with pressure in reservoir (4)
I believe the answer is:
sump
'reservoir' is the definition.
(I know that sump is a type of cistern)
'simple problem with pressure' is the wordplay.
'simple problem' becomes 'sum' (I've seen this in another clue).
'with' says to put letters next to each other.
'pressure' becomes 'p' (abbreviation).
'sum'+'p'='SUMP'
'in' is the link.
(Other definitions for sump that I've seen before include "oil well?" , "Into which liquids drain" , "lowest point of mine" , "Pit collecting water/oil" , "Pit receiving drainage" .)