Join in themselves (3)
I believe the answer is:
hem
'join' is the definition.
'in themselves' is the wordplay.
'in' says the answer is hidden in the clue.
'HEM' is hidden in the letters of 'themselves'.
(Other definitions for hem that I've seen before include "Express hesitation" , "Sewn border" , "Finished edge of a piece of cloth" , "A stitched border of cloth" , "Bottom of a garment" .)