Energy drink first, then grub cooked for one not responding to treatment (8)
I believe the answer is:
superbug
'one not responding to treatment' is the definition.
(a bug resistant to antibiotics)
'energy drink first then grub cooked' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mcu).
'drink' becomes 'sup' (I've seen this before).
'first' means one lot of letters go next to another.
'then' is a charade indicator (letters next to each other).
'cooked' indicates anagramming the letters.
'grub' with letters rearranged gives 'rbug'.
'e' put after 'sup' is 'supe'.
'supe'+'rbug'='SUPERBUG'
'for' is the link.
(Other definitions for superbug that I've seen before include "MRSA or C difficile" , "dangerous organism" , "Organism resistant to antibiotics" , "Resistant strain of bacteria" , "Microorganism resistant to antibiotics" .)