Watch pressure escape slowly (4)
I believe the answer is:
seep
'escape slowly' is the definition.
(to seep is to leak or escape slowly)
'watch pressure' is the wordplay.
'watch' becomes 'see' (see can mean to view or watch).
'pressure' becomes 'p' (abbreviation).
'see'+'p'='SEEP'
(Other definitions for seep that I've seen before include "Come out slowly" , "Leak gradually" , "Of liquid, ooze slowly out" , "Leak slowly, ooze" , "Trickle through slowly" .)