Ultimately confident PIN number opens security for temporary accommodation (4,3)
I believe the answer is:
tent peg
'security for temporary accommodation' is the definition.
(a tent peg secures a tent, a temporary structure)
'ultimately confident pin number opens' is the wordplay.
'ultimately confident' becomes 'tet' (I can't explain this - if you can you should believe this answer much more).
'pin' becomes 'peg' (peg is a kind of pin).
'number' becomes 'n' (n can represent any number).
'opens' is an insertion indicator (one set of letters makes an 'opening' in another).
'tet'+'peg'='tetpeg'
'tetpeg' going around 'n' is 'TENT-PEG'.
(Other definitions for tent peg that I've seen before include "Securer for marquee" , "Guy's hooked on this" , "Fastening for guy-rope" , "used to make a guy secure" , "Fastener for guy" .)