Notice pressure leak (4)
I believe the answer is:
seep
'leak' is the definition.
(I've seen this in another clue)
'notice pressure' is the wordplay.
'notice' becomes 'see' (notice can mean to spot or see).
'pressure' becomes 'p' (abbreviation).
'see'+'p'='SEEP'
(Other definitions for seep that I've seen before include "Ooze, percolate" , "Leak gradually" , "Flow or leak slowly" , "dribble away" , "Come out slowly" .)