Security pass on the side (10)
I believe the answer is:
collateral
'security' is the definition.
(collateral is a kind of security)
'pass on the side' is the wordplay.
'pass' becomes 'col' (col is a kind of pass).
'on the side' becomes 'lateral' (I've seen this before).
'col'+'lateral'='COLLATERAL'
(Other definitions for collateral that I've seen before include "running side by side" , "Secondary; parallel" , "Security for loan repayment; secondary" , "Guarantee" , "Money put up" .)