Something used by cook, as it happens, in modified olio (5,3)
I believe the answer is:
olive oil
'something used by cook' is the definition.
I can't tell whether this defines the answer.
'as it happens in modified olio' is the wordplay.
'as it happens' becomes 'live' (as in live broadcasting).
'in' indicates putting letters inside.
'modified' is an anagram indicator.
'olio' with letters rearranged gives 'ooil'.
'live' inserted into 'ooil' is 'OLIVE-OIL'.
Can you help me to learn more?
(Other definitions for olive oil that I've seen before include "Light yellow dressing pressed from Mediterranean tree fruits" , "Substance pressed from fruit" , "Pressed cooking ingredient" , "Ingredient of vinaigrette" , "Liquid used in cookery or salad dressings" .)