Queen tucks into elixir: a serving? (7)
I believe the answer is:
portion
'a serving?' is the definition.
('portion' can be a synonym of 'serving')
'queen tucks into elixir' is the wordplay.
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'tucks into' indicates putting letters inside.
'elixir' becomes 'potion' (I've seen this before).
'r' going within 'potion' is 'PORTION'.
(Other definitions for portion that I've seen before include "allotment" , "Share, helping" , "Piece of a whole" , "Part - share" , "Serving" .)