Customer has run for opening of sale, one that will last a long time (7)
I believe the answer is:
endurer
'customer has run for opening of sale one that' is the definition.
The definition suggests a singular noun which matches the answer.
'will last a long time' is the wordplay.
'will' says to put letters next to each other (I've seen this in other clues).
'last' becomes 'end' (I've seen this in another clue).
'a long time' becomes 'urer' (I can't explain this - if you can you should believe this answer much more).
'end'+'urer' is 'ENDURER'.
Can you help me to learn more?
(Another definition for endurer that I've seen is " one who's lasted".)