Endless surprise, working for major retailer (6)
I believe the answer is:
amazon
'retailer' is the definition.
(I know that Amazon is a online shop)
'endless surprise working for major' is the wordplay.
'endless' means to remove the last letter.
'surprise' becomes 'amaze' (amazing is a kind of surprising).
'working for major' becomes 'on' (I can't justify this - if you can you should give a lot more credence to this answer).
'amaze' with its final letter taken away is 'amaz'.
'amaz'+'on'='AMAZON'
Can you help me to learn more?
(Other definitions for amazon that I've seen before include "running through Peru, Colombia and Brazil" , "Great river of S America, or a tough woman" , "big river" , "Warlike female" , "Flower" .)