The most common and most likely answer to this crossword clue is the 14 letter word INTERPLANETARY
I've seen this crossword clue in The Guardian
I
N
T
E
R
P
L
A
N
E
T
A
R
Y
Buddy explains!
The straight part
We know that "between Mars and Venus, perhaps" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case between mars and venus, perhaps = interplanetary.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 INTERPLANETARY mean?
INTERPLANETARY adjective- Existing or occurring between planets