Making bubbly adult limit energy at first (8)
I believe the answer is:
aeration
'making bubbly' is the definition.
(to aerate something is to make it bubbly)
'adult limit energy at first' is the wordplay.
'adult' becomes 'a' (abbreviation).
'limit' becomes 'ration' (rationing is a kind of limiting).
'energy' becomes 'e' (physics symbol as in E=mca).
'at first' means one lot of letters go next to another.
'ration' after 'e' is 'eration'.
'a'+'eration'='AERATION'
(Other definitions for aeration that I've seen before include "The process of adding carbon dioxide to liquid under pressure" , "Mixing with air/gas" .)