Security device that could be testable (8)
I believe the answer is:
seatbelt
'security device' is the definition.
'seatbelt' can be an answer for 'device' (seatbelt is a kind of device). I'm not certain of the 'security' bit.
'could be testable' is the wordplay.
'could be' is an anagram indicator.
'testable' with letters rearranged gives 'SEATBELT'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for seatbelt that I've seen before include "Safety feature" , "compulsory 30 years ago" .)