Heating unit for the Royal Marines (5)
I believe the answer is:
therm
'heating unit' is the definition.
'therm' can be an answer for 'unit' (therm is a kind of unit). I am not certain of the 'heating' bit.
'the royal marines' is the wordplay.
'royal marines' becomes 'rm'.
'the'+'rm'='THERM'
'for' is the link.
Can you help me to learn more?
(Other definitions for therm that I've seen before include "Amount of heat" , "Unit of heat used in the UK as the basis of gas bills" , "Gasman's unit" , "Unit of heat and gas measure" , "Former gas supply unit" .)