The most common and most likely answer to this crossword clue is the 4 plus 3 letter word FLAT TOP
I've seen this crossword clue in Financial Times
F
L
A
T
T
O
P
Buddy explains!
The straight part
We know that "haircut" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case haircut = flat top.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 FLAT TOP mean?
FLAT TOP noun- A short haircut in which the hair is brushed straight up then cut flat across the top.
- An aircraft carrier.
- A type of stringed instrument, most often an acoustic guitar, with a flat top (as opposed to an archtop), with strings held in place with pins, and with a complex system of bracing struts on the top.