Explosion engulfs potassium I was cooking (6)
I believe the answer is:
baking
'cooking' is the definition.
(baking is a kind of cooking)
'explosion engulfs potassium i' is the wordplay.
'explosion' becomes 'bang' (I've seen this in another clue).
'engulfs' is an insertion indicator.
'potassium' becomes 'K' (K is the chemical symbol for potassium).
'k'+'i'='ki'
'bang' enclosing 'ki' is 'BAKING'.
'was' is the link.
(Other definitions for baking that I've seen before include "Making bread, cakes etc" , "Very hot; making bread" , "Making bread, cake or pastry" , "Extremely hot" , "Making, eg, cakes" .)