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Not on a length, need pluck (5)

Ross

I believe the answer is:

offal

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'need pluck' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'not on a length' is the wordplay.
'not on' becomes 'off' ('off' is the opposite to 'on').
'length' becomes 'l' (abbreviation in geometry).
'off'+'a'+'l'='OFFAL'

Can you help me to learn more?

(Other definitions for offal that I've seen before include "Heart, liver, kidney etc" , "Edible animal viscera" , "Meaty waste" , "Liver or tongue, for example" , "Edible innards" .)

I've seen this clue in The Hindu.
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