Conceal two aliens, one with no head (6)

I believe the answer is:

pocket

I'm a little stuck... Click here to teach me more about this clue!

'head' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for pocket that I've seen before include "Pouch; steal" , "Take for oneself" , "Pouch in garment" , "Electronic maths aid" , "Small isolated group" .)

I've seen this clue in The Telegraph.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.