Approaches concealed by nine arsonists (5)
I believe the answer is:
nears
'approaches' is the definition.
('near' can be a synonym of 'approach')
'concealed by nine arsonists' is the wordplay.
'concealed by' says the answer is hidden in the clue.
'NEARS' is hidden in the letters of 'nine arsonists'.
(Other definitions for nears that I've seen before include "Saner (anag.)" , "Draws close" , "advances" , "Uses snare to get closer" , "is approaching" .)