Wear armour regularly? That would be a mistake (5)
I believe the answer is:
error
'a mistake' is the definition.
('error' can be a synonym of 'mistake')
'wear armour regularly?' is the wordplay.
'regularly?' indicates alternate letters (regularly take one letter, leave next etc.).
'wear'+'armour'='weararmour'
The alternate letters of 'weararmour' are 'ERROR'.
'that would be' is the link.
(Other definitions for error that I've seen before include "Faux pas" , "heresy" , "wrongdoing" , "Way of operating" , "Fallacy, mistake" .)