Such a customer may be right, and argue about protecting Left (7)
I believe the answer is:
regular
'such a customer' is the definition.
(regular is a kind of customer)
'right and argue about protecting left' is the wordplay.
'right' becomes 'r' (common abbreviation).
'and' says to put letters next to each other.
'about' is an anagram indicator.
'protecting' is an insertion indicator.
'left' becomes 'l' (common abbreviation**).
'argue' anagrammed gives 'eguar'.
'eguar' going around 'l' is 'egular'.
'r'+'egular'='REGULAR'
'may be' acts as a link.
(Other definitions for regular that I've seen before include "Recurring in a uniform manner, say pulse" , "One present often" , "Career soldier" , "Customary; done frequently" , "In accordance with a fixed order" .)