Oracle 11.2.0.3.0 doesn't support APPLY in linq

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C#

LINQ

Oracle

i have a linq query where I try to do a groupby to obtain data distinct for a column field (RAGIONE_SOCIALE).
But when execute query, oracle gives me an error:
Oracle 11.2.0.3.0 doesn't support APPLY in linq

my linq query is:

Iquerable<VEVO_CLIENTI_SITI_STRUMENTI> query = from cliente in cont.VEVO_CLIENTI_SITI_STRUMENTI orderby cliente.RAGIONE_SOCIALE select cliente;

var vRes = (from clienteDef in query

select new ClienteFiltrato
{
RAGIONE_SOCIALE = clienteDef.RAGIONE_SOCIALE,
ID_CLIENTE = clienteDef.ID_CLIENTE,
ID_SITO =clienteDef.ID_SITO,
COD_SITO =clienteDef.COD_SITO,
COD_STRUMENTO = clienteDef.COD_STUMENTO,
DATA_DA = clienteDef.DA_DATA,
DATA_A = clienteDef.A_DATA
})                    .GroupBy(y=>y.RAGIONE_SOCIALE).Select(z=>z.FirstOrDefault());
                    return vRes.ToList();

how can i use groupby without having that error?
thanks a lot