CCTV, perhaps, almost always effective -- conceal weapon (12)
I believe the answer is:
surveillance
'cctv perhaps' is the definition.
'almost always effective conceal weapon' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'always effective' becomes 'sure'.
'conceal' becomes 'veil' (veiling is a kind of concealing**).
'weapon' becomes 'lance' (lance is a kind of weapon).
'sure' with its last letter taken off is 'sur'.
'sur'+'veil'+'lance'='SURVEILLANCE'
(Other definitions for surveillance that I've seen before include "Close observation" , "Covert observation" , "Watch" .)