Watch phosphorus leak out (4)
I believe the answer is:
seep
'leak out' is the definition.
(I've seen this in another clue)
'watch phosphorus' is the wordplay.
'watch' becomes 'see' (seeing is a kind of watching).
'phosphorus' becomes 'P' (P is the chemical symbol for phosphorus).
'see'+'p'='SEEP'
(Other definitions for seep that I've seen before include "Flow or leak slowly" , "Leak slowly, ooze" , "Come out slowly" , "Ooze, percolate" , "Leak through an opening" .)