Many a security device has a timer (5)
I believe the answer is:
clock
'a timer' is the definition.
(thesaurus)
'many a security device' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'a security device' becomes 'lock' (I've seen this before).
'c'+'lock'='CLOCK'
'has' acts as a link.
(Other definitions for clock that I've seen before include "Recording device" , "Timepiece" , "time" , "Flashy black-market dealer" , "Notice" .)