Welcome back and thanks for another great set of clues, Jolan. It’s a rare treat to have both a free Tuesday and your 5@5 to tackle.
Here is my stab at it.
1. Vault Merlin re-positioned between the ramparts (11)
Intervallum?
Anagram of “vault merlin” gives the word meaning the space between the ramparts.
2. Take advantage of race to make an uneventful journey in the US (4,3)
Milk run?
Take advantage of – “milk” and race – “run” gives the US phrase meaning an uneventful journey.
3. Popular male superintendent loses heart and joins leading tong as supplier of dope (9)
Informant?
Popular – “in”, male superintendent loses heart–“for(e)man”, leading tong – “t” , gives informant as “supplier of dope” as in information.
4. Beetle, for example, with bony extremities usually found under the hood in Oz? (5)
Carby?
Beetle, for example is “car”, bony extremities is “by” giving “carby” the informal Australian word for carburettor, which is “under the hood”.
5. Tilts without using jack then topples (5)
Ousts?
Tilts- jousts, without J for jack is “ousts”, or topples.