Keen customer might need this to access her online bank details? (4)
I believe the answer is:
acid
'keen' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for acid that I've seen before include "Alkali's opposite" , "Tart - keen - sort of house party" , "Caustic and biting" , "Sour - vitriolic" , "Drug, tart" .)