Foreach loop inside array

You use foreach to access the data, not define it.

Try this:

array(
    'label' => 'Assign to user',
    'desc' => 'Choose a user',
    'id' => $prefix.'client',
    'type' => 'radio'
    'options' => $clients
    )

If you need to change the structure of the data for 'options', do this before defining the primary array.

You cannot use the foreach in the definition of the array. You can however put the $clients variable in the array itself or you can foreach outside the array to build the array to be inserted at the options key

That's invalid syntax. You'd have to build the "parent" portions of the array first. THEN add in the sub-array stuff with the foreach loop:

$foo = array(
    'label' => 'Assign to user',
    'desc' => 'Choose a user',
    'id' => $prefix.'client',
    'type' => 'radio',
    'options' => array()
);

foreach ($clients as $user) {
    $foo['options'][] = array (  
        'label' => $user->user_login,  
        'value' => $user->user_login,
    );
}