The most common and most likely answer to this crossword clue is the 7 letter word CONSIST
I've seen this crossword clue in Financial Times
C
O
N
S
I
S
T
Buddy explains!

The straight part
We know that "to be made up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to be made up = consist.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 CONSIST mean?
CONSIST verb- To be.
- To exist.
- (with in) To be comprised or contained.
- (with of) To be composed, formed, or made up (of).
- A lineup or sequence of railroad carriages or cars, with or without a locomotive, that form a unit.