Going on, a part of a yard (5)
I believe the answer is:
afoot
'going on' is the definition.
(I've seen this before)
'a part of a yard' is the wordplay.
'part of a yard' becomes 'foot' ('foot' is part of 'yard').
'a'+'foot'='AFOOT'
(Other definitions for afoot that I've seen before include "In the pipeline" , "Progressing, in operation - like Sherlock Holmes's game" , "About to happen" , "Astir" , "Going on" .)