Duplicate a letter, at length, for a slippery customer (3)
I believe the answer is:
eel
'slippery customer' is the definition.
(I've seen this in another clue)
'duplicate a letter at length' is the wordplay.
'duplicate a letter' becomes 'ee' (I can't justify this - if you can you should give a lot more credence to this answer).
'at' means one lot of letters go next to another.
'length' becomes 'l' (abbreviation in geometry).
'ee'+'l'='EEL'
'for a' is the link.
Can you help me to learn more?
(Other definitions for eel that I've seen before include "Sea creature like conger" , "Proverbially slippery fish" , "Slippery swimmer (may be jellied)" , "Long thin fish" , "Slender fish" .)