Gas managed to overwhelm cook (5)
I believe the answer is:
radon
'gas' is the definition.
(I've seen this before)
'managed to overwhelm cook' is the wordplay.
'managed' becomes 'ran' (I've seen this before).
'to overwhelm' is an insertion indicator.
'cook' becomes 'do' (I've seen this in other clues).
'ran' going around 'do' is 'RADON'.
(Other definitions for radon that I've seen before include "Colourless gas, atomic number 86" , "Rare noble gas" , "A radioactive gas, symbol Rn" , "Gaseous element, Rn" , "Element, symbol Rn" .)