Observe phosphorus ooze (4)
I believe the answer is:
seep
'ooze' is the definition.
(I know that ooze can be written as seep)
'observe phosphorus' is the wordplay.
'observe' becomes 'see' (I've seen this in other clues).
'phosphorus' becomes 'P' (P is the chemical symbol for phosphorus).
'see'+'p'='SEEP'
(Other definitions for seep that I've seen before include "dribble away" , "Leak slowly, ooze" , "Trickle through slowly" , "Flow or leak slowly" , "Ooze, percolate" .)