Spray rust treated with phosphorus (5)
I believe the answer is:
spurt
'spray' is the definition.
(thesaurus)
'rust treated with phosphorus' is the wordplay.
'treated with' indicates anagramming the letters.
'phosphorus' becomes 'P' (P is the chemical symbol for phosphorus).
'rust' with its letters anagrammed with 'p' makes 'SPURT'.
(Other definitions for spurt that I've seen before include "Gush out suddenly" , "A short burst of activity" , "Short spell of increased speed" , "Jet, gush" , "Gush; sudden burst" .)