Rising gas beginning to drop a lot (2,3)
I believe the answer is:
no end
'a lot' is the definition.
(I've seen this before)
'rising gas beginning to drop' is the wordplay.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'gas' becomes 'neon' (I've seen this before).
'beginning to' indicates taking the first letters.
The initial letter of 'drop' is 'd'.
'neon' reversed gives 'noen'.
'noen'+'d'='NO END'
(Other definitions for no end that I've seen before include "Very much (unfinished?)" , "Enormously" , "Without limit" , "Very much (colloq.)" , "detailed?" .)