Explosion across dry valley (6)
I believe the answer is:
bottom
'valley' is the definition.
(thesaurus)
'explosion across dry' is the wordplay.
'explosion' becomes 'boom'.
'across' is an insertion indicator.
'dry' becomes 'tt' (abbreviation for teetotal).
'boom' placed around 'tt' is 'BOTTOM'.
(Other definitions for bottom that I've seen before include "foot" , "Lowest point - weaver" , "mechanic, for instance" , "Shakespearean - rear" , "Low point" .)