JQL - issues with parent belonging to particular epic

[Julz]

I need to find all the sub-tasks in my project that have parent in its turn belonging to the particular epic. Unfortunately sub-tasks do not have epic field, otherwise it would be easy. 

Thus, the tree-structure is like:
Epic --> Task --> Sub-task that I need to find.

Is it possible without plug-ins?

[Julz · Accepted solution]

Luckily I've found it myself, perhaps it might help anyone else.

issuetype = Sub-task AND issuekey in childIssuesOf(<epic_link>)

[Jack Brickey]

I may be mistaken but to the best of my knowledge childissueof () is not part of Jira OOTB. At least not in Cloud. I think, but not sure, that it is associated with this app - jql-search-extensions-for-jira-reports