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