Vegetable energy going into upped swan? (4)
I believe the answer is:
neep
'vegetable' is the definition.
(I know that neep is a type of cruciferous vegetable)
'energy going into upped swan?' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'going into' indicates putting letters inside.
'upped' says the letters should be written in reverse (in down clue, made to go up).
'swan?' becomes 'pen' (pen is a kind of swan).
'pen' written backwards gives 'nep'.
'e' put into 'nep' is 'NEEP'.
(Other definitions for neep that I've seen before include "vegetable in Scotland" , "Scots veg" , "Scots turnip" , "(Scottish) turnip" , "Root" .)