tag ‧ a ‧ long /ˈtæɡəlɒŋ $ -lɒːŋ/ BrE AmE noun [countable]
1 . British English something that is attached to and pulled behind something else:
The tagalong attaches to an adult’s bicycle.
2 . someone who goes somewhere with someone else:
We were tagalongs on my parents’ vacations.