Pass on one side for security (10)
I believe the answer is:
collateral
'for security' is the definition.
(collateral is a kind of security)
'pass on one side' is the wordplay.
'pass' becomes 'col' (col is a term for a mountain pass).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'one 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" , "Something pledged as security for a loan" , "damage?" , "Security for loan repayment; secondary" , "Money put up" .)