It extracts json structure with parsing string json and picking first element of array

jq-skeleton.sh

jq 'def r: walk(if type == "array" then if .[0] then [.[0]] else [] end else (fromjson? | r) // . end); r' a.json

# a.json
# {
#   "array": [1,2,3],
#   "nested_json": "{\"likes\": [1,2,3]}"
# }
#
# output
# {
#   "array": [
#     1
#   ],
#   "nested_json": {
#     "likes": [
#       1
#     ]
#   }
# }