The most common and most likely answer to this crossword clue is the 6 letter word HALLOO
H
A
L
L
O
O
Buddy explains!

The straight part
We know that "R/E" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case r/e = halloo.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does HALLOO mean?
HALLOO noun- A shout of halloo.
- To shout halloo.
- To encourage with shouts; to egg (someone) on.
- To chase with shouts or outcries.
- To call or shout to; to hail.
- To shout (something).
- Used to greet someone, or to catch their attention.
- Used in hunting to urge on the pursuers.