I fail to enlighten, excite or stretch (8)

I believe the answer is:

lengthen

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'stretch' is the definition.
(I know that lengthen is the is a more specific form of the action of stretch.)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for lengthen that I've seen before include "Make or become longer" , "Spin out" , "Elongate" , "Extend or draw out" , "Protract" .)

I've seen this clue in the Sydney Morning Herald.

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