First in the field, evidently long established (6)

I believe the answer is:

oldest

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'first' is the definition.
(similar in meaning)

'in the field evidently long established' is the wordplay.
'in the' is an insertion indicator.
'field evidently' becomes 'od' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'long' becomes 'l'.
'established' becomes 'est' (abbreviated used in shop signs).
'od' enclosing 'l' is 'old'.
'old'+'est'='OLDEST'

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(Other definitions for oldest that I've seen before include "Least young" , "Let's do! (anag.)" , "Most aged" , "Eldest" , "Most ancient (O, not E)" .)

I've seen this clue in the Evening Standard.

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