Petition son beforehand to use aerosol (5)
I believe the answer is:
spray
'aerosol' is the definition.
(I've seen this before)
'petition son beforehand' is the wordplay.
'petition' becomes 'pray' (I've seen this before).
'son' becomes 's' (genealogical abbreviation for son).
'beforehand' is a charade indicator (letters next to each other).
'pray' put after 's' is 'SPRAY'.
'to use' is the link.
(Other definitions for spray that I've seen before include "Treat with insecticide etc" , "Spindrift, flower bunch" , "Bunch of flowers" , "Disperse in tiny drops" , "Squirt water" .)