The most common and most likely answer to this crossword clue is the 5 letter word TOPOS
I've seen this crossword clue in The Times
T
O
P
O
S
Buddy explains!

The straight part
We know that "Basic concept" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case basic concept = topos.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 TOPOS mean?
TOPOS noun- A literary theme or motif; a rhetorical convention or formula.
- An elementary topos
- A Grothendieck topos