In radio telephony, I have a security device! (5)
I believe the answer is:
rivet
'a security device' is the definition.
'rivet' can be an answer for 'device' (rivet is a kind of device). I'm not certain of the 'a security' bit.
'in radio telephony i have' is the wordplay.
'in' indicates putting letters inside.
'radio telephony' becomes 'rt' (this could be a standard abbreviation which I don't know about).
'i have' becomes 'I've' (contraction of 'I have').
'rt' going around 'ive' is 'RIVET'.
Can you help me to learn more?
(Other definitions for rivet that I've seen before include "Arrest" , "Fasten -- engross" , "Metal pin; engross" , "Bolt for metal plates" , "Steel-plate pin" .)