Same retail carried out inside a triangle (11)
I believe the answer is:
equilateral
'a triangle' is the definition.
(equilateral triangle is a kind of triangle)
'same retail carried out' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'retail' is 'ilater' which is present in the answer.
'same' could be 'equal' (associated in meaning) and 'equal' is present in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'inside' is the link.
Can you help me to learn more?
(Other definitions for equilateral that I've seen before include "having every side the same length" , "Like regular shapes" , "Like diamonds, for example" , "Having all sides of equal length" , "like some figures on plane" .)