Security men only breaking tool (7)
I believe the answer is:
hostage
'security' is the definition.
(I've seen this before)
'men only breaking tool' is the wordplay.
'men only' becomes 'stag' (I've seen this in another clue).
'breaking' is an insertion indicator.
'tool' becomes 'hoe' (hoe is a kind of tool).
'stag' placed within 'hoe' is 'HOSTAGE'.
(Other definitions for hostage that I've seen before include "Person held by kidnappers" , "Person taken as security" , "One held captive" , "Prisoner kept as security" , "Person held by kidnapper for ransom" .)