For money, becoming the lawful guardian of (5,6)
I believe the answer is:
legal tender
'for money' is the definition.
(I've seen this before)
'the lawful guardian of' is the wordplay.
'the lawful' becomes 'legal' (I've seen this in another clue).
'guardian of' becomes 'tender' (I am not sure about this - if you are sure you should believe this answer much more).
'legal'+'tender'='LEGAL TENDER'
'becoming' acts as a link.
Can you help me to learn more?
(Other definitions for legal tender that I've seen before include "Official currency" , "Valid currency" , "... [HALF CROWNS] no longer this" , "coins?" , "Money which must be accepted as payment" .)