Total pressure in reservoir (4)
I believe the answer is:
sump
'reservoir' is the definition.
(sump is a kind of reservoir)
'total pressure' is the wordplay.
'total' becomes 'sum' (synonyms).
'pressure' becomes 'p' (abbreviation).
'sum'+'p'='SUMP'
'in' acts as a link.
(Other definitions for sump that I've seen before include "Hole collecting water for pump" , "Engine oil reservoir" , "Crankcase reservoir" , "Pit collecting water/oil" , "Oil store" .)