Crossword Genius

Wear armour occasionally? That's a mistake (5)

Ross

I believe the answer is:

error

'that's a mistake' is the definition.
(synonyms)

'wear armour occasionally?' is the wordplay.
'occasionally?' means one should take alternating letters (I've seen 'occasionally' mean this).
'wear'+'armour'='weararmour'
The alternate letters of 'weararmour' are 'ERROR'.

(Other definitions for error that I've seen before include "Bloomer, blunder" , "Faulty judgment" , "1 down" , "Way of operating" , "Gaffe" .)

I've seen this clue in The Times.
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