Crossword Genius

Reach instruction for knitting a giant sweater? (4,1,4,3)

Ross

I believe the answer is:

make a long arm

I'm a little stuck... Click here to teach me more about this clue!

'giant sweater?' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'reach instruction for knitting a' is the wordplay.
I cannot quite understand how this works, but
'reach' could be 'make' (synonyms) and 'make' is found in the answer.
'a' is found in the answer.
The remaining letters 'longarm' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Another definition for make a long arm that I've seen is " Grab everything you can!".)

I've seen this clue in The Independent.
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