For example, join in with this position (6)
I believe the answer is:
stance
'this position' is the definition.
(stance is a kind of position)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for stance that I've seen before include "Standpoint" , "One's physical or mental attitude" , "driver preparing to strike" , "Posture assumed" , "Attitude - taken by a golfer?" .)