Idiot you texted operated cellphone lacking core security (9)
I believe the answer is:
assurance
'security' is the definition.
(assurance is a kind of security)
'idiot you texted operated cellphone lacking core' is the wordplay.
'idiot' becomes 'ass' (I've seen this before).
'you texted' becomes 'u' (text abbreviation for 'you').
'operated' becomes 'ran' ('run' can be a synonym of 'operate'**).
'lacking core' suggests removing the centre.
'cellphone' with its middle taken out is 'ce'.
'ass'+'u'+'ran'+'ce'='ASSURANCE'
(Other definitions for assurance that I've seen before include "Word given" , "Confidence, certainty" , "Confidence; security" , "Confidence, composure" , "Confidence (in abilities)" .)