Crossword Genius

Lettuce variety firm and small (3)

Ross

I believe the answer is:

cos

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

'firm and small' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'and small' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'co'+'s'='COS'

Can you help me to learn more?

(Other definitions for cos that I've seen before include "Casual as" , "something of a salad" , "Senior officers" , "in the Dodecanese?" , "leaves > function" .)

I've seen this clue in The Telegraph.
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