DECLARE
  l_issue_date a_secs.issue_date%type;
  l_issued_volume NUMBER;
BEGIN
  SELECT a.issue_date
    INTO l_issue_date
    FROM a_secs a
    WHERE some_condition_here;  -- in order to get one and only one value

  IF l_issue_date <= sysdate    -- sysdate is used just as an example; I don't know what "?As At Date" is
  THEN 
     l_issued_volume := l_issue_date;
  ELSE
     l_issued_volume := 0;
  END IF;
END;

IF a = 'ABC' OR b = 'EFG'
THEN ...

IF function_1 (some_parameter) = 'ABC' THEN
   IF function_2 (another_parameter) = 'EFG' THEN
      IF function_e (third_parameter) = 'HIJ' THEN
         var_x := 0;
      END IF;
   END IF;
END IF;