A certain military group getting some heat (5)
I believe the answer is:
therm
'some heat' is the definition.
(Therm is a unit of heat energy)
'a certain military group getting' is the wordplay.
'a certain' becomes 'the' (definite article - refers to a certain object).
'military group' becomes 'RM' (Royal Marines).
'getting' means one lot of letters go next to another.
'the'+'rm'='THERM'
(Other definitions for therm that I've seen before include "Unit of heat as in gas supply" , "Unit of measurement of heat" , "Indicator of heat" , "Amount of heat" , "it measures heat" .)