Kind agent leaves missing letters (6)

I believe the answer is:

gentle

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

'kind' is the definition.
(I've seen this in another clue)

'agent leaves missing letters' is the wordplay.
'GENTLE' is hidden within the letters.
However, I'm unsure how this is indicated.

Can you help me to learn more?

(Other definitions for gentle that I've seen before include "'Do not go . . . . . . into that good night' (Dylan Thomas)" , "Moderate, quiet; a maggot" , "Merciful -- grub" , "Not harsh or violent" , "Clement" .)

I've seen this clue in The Times.

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