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' is the 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" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app