The most common and most likely answer to this crossword clue is the 6 letter word MANIOC
I've seen this crossword clue in Financial Times
M
A
N
I
O
C
More clues leading to the result MANIOC
Buddy explains!

The straight part
We know that "starch" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case starch = manioc.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 MANIOC mean?
MANIOC noun- The tropical plant Manihot esculenta, from which cassava and tapioca are prepared.
- Cassava root, eaten as a food.
- A food starch prepared from the root.