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