The most common and most likely answer to this crossword clue is the 3 letter word RIB
R
I
B
Buddy explains!

The straight part
We know that "alternately K and P" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case alternately k and p = rib.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 RIB mean?
RIB noun- Any of a series of long curved bones occurring in 12 pairs in humans and other animals and extending from the spine to or toward the sternum
- A part or piece, similar to a rib, and serving to shape or support something
- A cut of meat enclosing one or more rib bones
- Any of several curved members attached to a ship's keel and extending upward and outward to form the framework of the hull
- Any of several transverse pieces that provide an aircraft wing with shape and strength
- A long, narrow, usually arched member projecting from the surface of a structure, especially such a member separating the webs of a vault
- A raised ridge in knitted material or in cloth
- The main, or any of the prominent veins of a leaf
- A teasing joke
- A single strand of hair.
- A stalk of celery.
- (archaic, literary, humorous) A wife or woman.
- To shape, support, or provide something with a rib or ribs.
- To tease or make fun of someone in a good-natured way.
- To enclose, as if with ribs, and protect; to shut in.
- To leave strips of undisturbed ground between the furrows in ploughing (land).
- Hound's-tongue (Cynoglossum officinale).
- Costmary (Tanacetum balsamita).
- Watercress (Nasturtium officinale).