Even bits of basil and oil spoil salad dressing (5)
I believe the answer is:
aioli
'salad dressing' is the definition.
'aioli' can be an answer for 'dressing' (type of condiment or dressing). I am not sure about the 'salad' bit.
'even bits of basil and oil spoil' is the wordplay.
'even bits of' means one should take alternating letters (letters in even-numbered positions are used).
'and' says to put letters next to each other.
'spoil' is an anagram indicator.
The alternate letters of 'basil' are 'ai'.
'oil' with letters rearranged gives 'oli'.
'ai'+'oli'='AIOLI'
Can you help me to learn more?
(Other definitions for aioli that I've seen before include "Mayo, in part?" , "Garlic mayo" , "A lamb joint; driven clouds" , "garlic dip" , "Sort of mayonnaise" .)