Heard some pressure in drain (4)
I believe the answer is:
sump
'drain' is the definition.
(I've seen this before)
'heard some pressure' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'pressure' becomes 'p' (abbreviation).
'some' sounds like 'sum'.
'sum'+'p'='SUMP'
'in' acts as a link.
(Other definitions for sump that I've seen before include "Pit collecting water/oil" , "Crankcase reservoir" , "Pit receiving drainage" , "Water collector" , "It receives the engine oil" .)