Let’s cook with additive – that’s hard (5)
I believe the answer is:
steel
'hard' is the definition.
'let's cook with additive' is the wordplay.
'cook' indicates anagramming the letters.
'with' indicates putting letters inside.
'additive' becomes 'e' (I can't justify this - if you can you should believe this answer much more).
'lets' anagrammed gives 'stel'.
'stel' going around 'e' is 'STEEL'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for steel that I've seen before include "nerve" , "Hard iron alloy" , "Iron/carbon alloy" , "Make hard and unfeeling" , "The staying power" .)