To initialize field symbols you should use keyword UNASSIGN instead of CLEAR.
And you can not use field symbol <fst_bsis_line> directly in the select statement. Instead put it in an internal table and then assign the contents to the FS as below:
tables: bsis.
types: begin of it_bsis_line,
zuonr like bsis-zuonr,
belnr like bsis-belnr,
end of it_bsis_line.
data: it_bsis type table of it_bsis_line,
lv_i type i.
field-symbols <fst_bsis_line> type any table.
field-symbols <fs> like line of it_bsis.
UNASSIGN <fst_bsis_line>.
lv_i = 1.
select * up to 10 rows from bsis into CORRESPONDING FIELDS
OF table it_bsis where zuonr ne ''.
ASSIGN it_bsis to <fst_bsis_line>.
loop at <fst_bsis_line> assigning <fs>.
write:/ lv_i,' - ', <fs>-zuonr, ' - ', <fs>-belnr.
lv_i = lv_i + 1.
endloop.
For further details refer: