See an outsize lettuce (3)

I believe the answer is:

cos

'lettuce' is the definition.
(cos is a kind of lettuce)

'see an outsize' is the wordplay.
'see' becomes 'c' (the word for the letter, according to Chambers).
'an outsize' becomes 'os'.
'c'+'os'='COS'

(Other definitions for cos that I've seen before include "Mozart fidelity-test opera" , "leaves > function" , "Greek island (alternative spelling): Trig abbrev!" , "foreign island" , "often associated with sin" .)

I've seen this clue in the Evening Standard.

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