Doddered along, mixed up (8)

I believe the answer is:

withered

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'doddered' is the definition.
(thesaurus)

'along mixed up' is the wordplay.
'along' becomes 'with' (synonyms).
'mixed' becomes 'dere' (I am not sure about this - if you are sure you should believe this answer much more).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'dere' written backwards gives 'ered'.
'with'+'ered'='WITHERED'

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(Other definitions for withered that I've seen before include "Lost freshness and dried up" , "declined" , "like Keats's sedge?" , "Dried and shrivelled up" , "Wrinkled by shrinkage from age" .)

I've seen this clue in The Telegraph.

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