There is no "Magic" way to extract the first element only.
My suggestion would be to copy the elements whose name start with "1." to another array as in:
json$="[{'name':'Alice','type':'A','balance':'123.45'},{'name':'Bob','type':'A','balance':'6543.21','extra-field':'1','extra2':'2'}]"
dim load array${all}=json$
dim one$
for k$ index array${all}
if mid(k$,1,2)="1." \
then one$[k$(3)]=array$[k$]
next k$
json1$=dim(list one${all})
print json1$
->run
{"name":"Alice","type":"A","balance":"123.45"}