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,
);
}