Envious: regularly seen filling up still (9)

I believe the answer is:

resentful

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'envious' is the definition.
(thesaurus)

'regularly seen filling up still' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'filling' means one lot of letters goes inside another.
'up still' becomes 'restful' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The alternate letters of 'seen' are 'en'.
'en' put inside 'restful' is 'RESENTFUL'.

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(Other definitions for resentful that I've seen before include "Embittered - nurse left (anag)" , "Feeling bitterness" , "Tiny wave" , "Bitter" , "Taking ill" .)

I've seen this clue in The Guardian.

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