The Marines forming a unit (5)
I believe the answer is:
therm
'a unit' is the definition.
(therm is a kind of unit)
'the marines' is the wordplay.
'marines' becomes 'rm' (abbreviation for 'Royal Marines').
'the'+'rm'='THERM'
'forming' acts as a link.
(Other definitions for therm that I've seen before include "Unit of heat as in gas supply" , "A hot unit" , "Unit of measurement of heat" , "it measures heat" , "Indicator of heat" .)