Nothing outside area is secure (4)
I believe the answer is:
nail
'is secure' is the definition.
(I've seen this before)
'nothing outside area' is the wordplay.
'nothing' becomes 'nil' ('nil' can be a synonym of 'nothing').
'outside' indicates putting letters inside.
'area' becomes 'a' (maths abbreviation).
'nil' placed around 'a' is 'NAIL'.
(Other definitions for nail that I've seen before include "Brad or tack" , "Fasten - detect and expose" , "Digital plate" , "it needs hammering home" , "Amall metal spike" .)