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 an anagram.
'phosphorus' becomes 'p' ('P' can be a synonym of 'phosphorus').
'rust' with its letters mixed up 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" .)