Cook special meal with a dash of marmite (5)
I believe the answer is:
steam
'cook' is the definition.
(steaming is a kind of cooking)
'special meal with a dash of marmite' is the wordplay.
'special' becomes 's' (common abbreviation as in Special Air Service).
'meal' becomes 'tea' (tea is a kind of meal).
'with' means one lot of letters go next to another.
'a dash of' says to take the initial letters.
The first letter of 'marmite' is 'm'.
's'+'tea'+'m'='STEAM'
(Other definitions for steam that I've seen before include "Source of energy" , "Momentum - boiled water" , "Form of water" , "A lot of hot air" , "It's water mist, mates" .)