WITH
s AS (
SELECT
  c.[id_client],
  ss.[service],
  [rn] = ROW_NUMBER() OVER ( 
    PARTITION BY 
      c.[id_client],
      CASE
        WHEN s.[somewhere] = 'somehow' THEN 1
        WHEN s.[somewhere] = 'somehow2' THEN 2
        ELSE 999
      END )
FROM
  clients c
  LEFT JOIN [services] s ON (
        s.[id_client] = c.[id_client] )
)
SELECT
  [id_client],
  [service]
FROM
  s
WHERE
  [rn] = 1

  [rn] = ROW_NUMBER() OVER ( 
    PARTITION BY 
      c.[id_client]
    ORDER BY
      CASE
        WHEN s.[somewhere] = 'somehow' THEN 1
        WHEN s.[somewhere] = 'somehow2' THEN 2
        ELSE 999
      END )

Select c.Id, s.service
from dbo.clients c
outer apply dbo.Function(c.id) s 

select c.*
	,s.*
from clients as c
  outer apply (
     select top(1) with ties
		s.*
	 from services as s 
	 where s.client_id = c.id
	 order by row_number() over (partition by s.client_id order by
				case
		        when s.[somewhere] = 'somehow' then 1
		        when s.[somewhere] = 'somehow2' then 2
				else 999	end
				)
	 ) as c