Regular errands made before returning short suit that only covers part of arm (6)
I believe the answer is:
radius
'part of arm' is the definition.
(I know that radius is part of forearm)
'regular errands made before returning' is the wordplay.
'regular' means one should take alternating letters.
'made before' becomes 'sui' (I can't justify this - if you can you should give a lot more credence to this answer).
'returning' shows that the letters should be reversed in order.
The alternate letters of 'errands' are 'rad'.
'sui' backwards is 'ius'.
'rad'+'ius'='RADIUS'
'short suit that only covers' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for radius that I've seen before include "Half a circle's diameter" , "Bone - half of diameter" , "Bone of forearm - distance from centre" , "Line from centre to edge of a circle" , "Part of build-up of arms" .)