Wear armour regularly showing flaw (5)
I believe the answer is:
error
'flaw' is the definition.
(flaw can mean a mistake or imperfection)
'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'.
'showing' acts as a link.
(Other definitions for error that I've seen before include "Slip-up" , "Mistake or mishap" , "Bug" , "Miscarriage of justice" , "wrongdoing" .)