The most common and most likely answer to this crossword clue is the 5 letter word FREAK
I've seen this crossword clue in The Times
F
R
E
A
K
Buddy explains!

The straight part
We know that "Rare" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rare = freak.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 FREAK mean?
FREAK noun- A sudden change of mind
- Someone or something that is markedly unusual or unpredictable.
- A hippie.
- A drug addict.
- (of a person) A nonconformist, especially in appearance, social behavior, sexual orientation, gender identity, and/or business practices; an oddball, especially in physiology (i.e., "circus freak"); unique, sometimes in a displeasing way.
- A person whose physique has grown far beyond the normal limits of muscular development; often a bodybuilder weighing more than 120 kilos (260 pounds).
- An enthusiast, or person who has an obsession with, or extreme knowledge of, something.
- (sometimes affectionate) A very sexually perverse individual.
- A streak of colour; variegation.
- To make greatly distressed and/or a discomposed appearance
- To be placed or place someone under the influence of a psychedelic drug
- To streak; to variegate
- To experience reality withdrawal, or hallucinations (nightmarish), to behave irrational or unconventional due to drug use.
- To react extremely or irrationally, usually under distress or discomposure
- Strange, weird, unexpected
- A man, particularly a bold, strong, vigorous man.
- A fellow; a petulant young man.