The most common and most likely answer to this crossword clue is the 4 letter word SOUS
I've seen this crossword clue in The Times
S
O
U
S
Buddy explains!

The straight part
We know that "Undercook" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case undercook = sous.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 SOUS mean?
SOUS noun- An old French copper coin equal to one twentieth of a livre or twelve deniers; one sou is to the livre as one shilling is to the pound.
- Cent; pocket money.
- A thing of the smallest value; a whit; a jot.
- An old French copper coin equal to one twentieth of a livre or twelve deniers; one sou is to the livre as one shilling is to the pound.
- Cent; pocket money.
- A thing of the smallest value; a whit; a jot.
- Short for sous-chef.
- An old French copper coin equal to one twentieth of a livre or twelve deniers; one sou is to the livre as one shilling is to the pound.
- Cent; pocket money.
- A thing of the smallest value; a whit; a jot.
- Short for sous-chef.