Rulers incorporating one metre units (9)
I believe the answer is:
regiments
'units' is the definition.
(regiment is a kind of unit)
'rulers incorporating one metre' is the wordplay.
'rulers' becomes 'regents' (regent is a kind of ruler**).
'incorporating' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'metre' becomes 'm' ('m' can be a synonym of 'metre').
'i'+'m'='im'
'regents' placed around 'im' is 'REGIMENTS'.
(Other definitions for regiments that I've seen before include "Organises" , "groups of soldiers" , "fighting units" .)