Move in the manner of a jackass joining a companion (6)
I believe the answer is:
twitch
'move' is the definition.
(twitching is a kind of moving)
'manner of a jackass joining a companion' is the wordplay.
'manner of a jackass joining' becomes 'twit' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a companion' becomes 'ch' (Companion of Honour).
'twit'+'ch'='TWITCH'
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for twitch that I've seen before include "nervous habit" , "Uncontrolled, jerky motion" , "involuntary response" , "jump" , "by implication, is to look for birds" .)