Holds up for days, outwardly (6)

I believe the answer is:

delays

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'holds up' is the definition.
(I've seen this before)

'for days outwardly' is the wordplay.
'for' becomes 'el' (I can't explain this - if you can you should give a lot more credence to this answer).
'outwardly' indicates putting letters inside.
'el' placed within 'days' is 'DELAYS'.

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(Other definitions for delays that I've seen before include "Slows down, holds back" , "Lags" , "Hinders; puts off" , "Retards or procrastinates" , "Stays or postponements" .)

I've seen this clue in the Evening Standard.

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