Top side beaten for security (7)
I believe the answer is:
deposit
'security' is the definition.
(I know that deposit is a type of security)
'top side beaten' is the wordplay.
'top side' becomes 'topside' (synonyms).
'beaten' is an anagram indicator (letters beaten up or destroyed).
'topside' anagrammed gives 'DEPOSIT'.
'for' acts as a link.
(Other definitions for deposit that I've seen before include "part-payment" , "Lay down" , "Hoard" , "Part payment in advance to secure goods" , "Put into, bank account say" .)