Sound investment may be shown in court (5)
I believe the answer is:
guilt
'may be shown in court' is the definition.
'sound investment' is the wordplay.
'sound' indicates a 'sounds like' (homophone) clue.
'investment' becomes 'gilt' (type of UK fixed-interest security).
'gilt' sounds like 'GUILT'.
(Other definitions for guilt that I've seen before include "The fact of having done wrong" , "State of having committed offence" , "(Shame for) criminality" , "Remorse, feeling responsible for an offence or action" , "(Feeling induced by) wrongdoing" .)