How to check/query only the first element of a nested field?

Elastic Stack Elasticsearch
1

Elasticsearch version ( bin/elasticsearch --version ):
5.6.11
Plugins installed :

JVM version ( java -version ):
openjdk version "1.8.0_222"
OpenJDK Runtime Environment (build 1.8.0_222-8u222-b10-1ubuntu1~16.04.1-b10)
OpenJDK 64-Bit Server VM (build 25.222-b10, mixed mode)

OS version ( uname -a if on a Unix-like system):
Linux myserver 4.4.0-131-generic #157-Ubuntu SMP Thu Jul 12 15:51:36 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Description of the problem including expected versus actual behavior :
I have a nested field in my index which holds the most recent element at the top (index 0), how could i enforce a query to ONLY check for this said first element and not query the rest of the elements in said nested field?

E.g. :

document with field "nested"

{
propx: "hi",
mynested: { 
[
  firs_el: "valueY",
  second_el: 123,
],
[
  firs_el: "dontmatter",
  second_el: 666,
],
}

in the example above I would like to query and check if the first element matches my criteria, and if it does i want to return the whole document

Thanks in advance

note: this concerns just the matching, not the returned results