Australian runner in team, sub regularly used (3)
I believe the answer is:
emu
'australian runner' is the definition.
(bird which runs)
'team sub regularly used' is the wordplay.
'regularly used' indicates alternate letters (letters selected on a regular basis).
'team'+'sub'='teamsub'
The alternating letters of 'teamsub' are 'EMU'.
'in' is the link.
(Other definitions for emu that I've seen before include "Physics unit" , "Bird of tremulous kind" , "unit of measurement" , "Aussie runner" , "Ratite" .)