Round up, with a comrade (3)
I believe the answer is:
pal
'a comrade' is the definition.
(I've seen this in other clues)
'round up' is the wordplay.
'round' becomes 'lap' (as in a round or lap of a park).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'lap' written backwards gives 'PAL'.
'with' is the link.
(Other definitions for pal that I've seen before include "Bud" , "Mate, informal friend" , "Brother" , "Buddy, chum" , "Close friend" .)