The most common and most likely answer to this crossword clue is the 4 letter word SOLE
I've seen this crossword clue in The Times
S
O
L
E
Buddy explains!
The straight part
We know that "Only one/flatfish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case only one/flatfish = sole.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 SOLE mean?
SOLE noun- A wooden band or yoke put around the neck of an ox or cow in the stall.
- A pond or pool; a dirty pond of standing water.
- To pull by the ears; to pull about; haul; lug.
- Only
- Unmarried (especially of a woman); widowed.
- The bottom or plantar surface of the foot.
- (footwear) The bottom of a shoe or boot.
- The foot itself.
- Solea solea, a flatfish of the family Soleidae.
- The bottom or lower part of anything, or that on which anything rests in standing.
- The seat or bottom of a mine; applied to horizontal veins or lodes.
- To put a sole on (a shoe or boot)