Formidable fellows giving outstanding service (8)

I believe the answer is:

menacing

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'formidable' is the definition.
(thesaurus)

'fellows giving outstanding service' is the wordplay.
'fellows' becomes 'men' (fellow can mean a man).
'giving outstanding service' becomes 'acing' (I can't justify this - if you can you should believe this answer much more).
'men'+'acing'='MENACING'

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(Other definitions for menacing that I've seen before include "Scary" , "Suggesting danger" , "Intimidating" , "Ominous" , "Threatening" .)

I've seen this clue in The Globe and Mail.

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