Group by checks constraints are possible?
You can use a unique index for that. Example:

create unique index one_leader_per_team
  on members 
    (case when IS_LEADER = 'Y' then TEAM_ID else null end) ;


What it does is store team_id - in the index - only for those rows that have IS_LEADER = 'Y' and nulls in the rest of the rows  The UNIQUE ensures that have only one row per team with 'Y' and the nulls are ignored. 

(I think that up to 11 and older version, Oracle doesn't store the nulls at all in the index, so the index is more space efficient - not sure if this has changed in version 12). That means that the Oracle index is equivalent to a filtered/partial index in other DBMS (SQL Server, PostgreSQL), i.e.:

-- this would work in SQL Server, Postgres 
create unique index one_leader_per_team
  on members (TEAM_ID)
  where (IS_LEADER = 'Y') ;


Test in dbfiddle.uk:




create table members
  ( MEMBER_ID integer not null primary key,
    TEAM_ID integer not null,
    NAME varchar(64) not null,
    IS_LEADER char(1)  default 'Y' not null check (IS_LEADER in ('Y','N'))
  ) ;

  
  





create unique index one_leader_per_team
  on members 
    (case when IS_LEADER = 'Y' then TEAM_ID end) ;

  
  





insert all
  into members values (1, 1, 'Alex', 'Y')
  into members values (2, 1, 'Bill', 'N')
  into members values (3, 1, 'Cath', 'N')
  into members values (4, 2, 'Diana', 'N')
  into members values (5, 2, 'Elsa', 'Y')
select * from dual ;



5 rows affected






insert all
  into members values (6, 3, 'Fay', 'Y')
  into members values (7, 3, 'George', 'Y')
select * from dual ;



ORA-00001: unique constraint (FIDDLE_TITLKRLCAFRLTVCBXZVI.ONE_LEADER_PER_TEAM) violated






insert into members values (8, 1, 'Harry', 'Y');



ORA-00001: unique constraint (FIDDLE_TITLKRLCAFRLTVCBXZVI.ONE_LEADER_PER_TEAM) violated






select * from members ;



MEMBER_ID | TEAM_ID | NAME  | IS_LEADER
--------: | ------: | :---- | :--------
        1 |       1 | Alex  | Y        
        2 |       1 | Bill  | N        
        3 |       1 | Cath  | N        
        4 |       2 | Diana | N        
        5 |       2 | Elsa  | Y        



db&lt;>fiddle here