Happening to use an imperial measure of length (5)
I believe the answer is:
afoot
'happening' is the definition.
('something's afoot' means 'something's happening')
'an imperial measure of length' is the wordplay.
'an' becomes 'a'.
'imperial measure of length' becomes 'foot' (foot is a unit of length).
'a'+'foot'='AFOOT'
'to use' is the link.
(Other definitions for afoot that I've seen before include "Currently in progress" , "Stirring" , "Under way (like hunted game?)" , "Brewing" , "Happening" .)