Young father's letter to Grecian (6)

I believe the answer is:

lambda

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'letter to grecian' is the definition.
I can't judge whether this definition defines the answer.

'young father's' is the wordplay.
'young' becomes 'lamb' (lamb is a kind of young).
'father' becomes 'da' (da can mean dad).
'lamb'+'da'='LAMBDA'

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(Other definitions for lambda that I've seen before include "Greek letter L" , "character from Greece" , "part of the skull" , "Steps taken" , "Foreign character" .)

I've seen this clue in The Independent.

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