Last place as explosion envelops races (6)
I believe the answer is:
bottom
'last place' is the definition.
(eg at the bottom of a league table)
'explosion envelops races' is the wordplay.
'explosion' becomes 'boom'.
'envelops' means one lot of letters goes inside another (I've seen this in other clues).
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'boom' going around 'tt' is 'BOTTOM'.
'as' acts as a link.
(Other definitions for bottom that I've seen before include "mechanic, for instance" , "Athenian weaver (AMND)" , "Lowest part of anything" , "Weaver < 's backside" , "foot" .)