Low and trimmed-down portable night vision aid (9)

I believe the answer is:

moonlight

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

'night vision aid' is the definition.
I can't tell whether this defines the answer.

'low and trimmed-down portable' is the wordplay.
'low' becomes 'moo' (both can mean to make a cow noise).
'trimmed' means to look at the middle letters (the external letters are trimmed from the word).
'down portable' becomes 'light' (I've seen this before. I am not sure about the 'down' bit.).
The middle letter of 'and' is 'n'.
'moo'+'n'+'light'='MOONLIGHT'

Can you help me to learn more?

(Other definitions for moonlight that I've seen before include "work illegally" , "Reflected illumination" , "Do extra work" , "Natural (and romantic?) illumination" , "Steerer of boat" .)

I've seen this clue in The Guardian.

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