Security firm fifty-fifty about dead gunmen (10)
I believe the answer is:
collateral
'security' is the definition.
(collateral is a kind of security)
'firm fifty-fifty about dead gunmen' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'fifty-fifty' becomes 'll' (L is 50 in Roman numerals).
'about' is an insertion indicator.
'dead' becomes 'late' (similar in meaning).
'gunmen' becomes 'ra'.
'late'+'ra'='latera'
'll' enclosing 'latera' is 'llateral'.
'co'+'llateral'='COLLATERAL'
(Other definitions for collateral that I've seen before include "subsidiary" , "Security for loan repayment; secondary" , "Money put up" , "damage?" , "running side by side" .)