Take a cold mushroom and tomato starter (6)
I believe the answer is:
accept
'take' is the definition.
('accept' can be a synonym of 'take')
'a cold mushroom and tomato starter' is the wordplay.
'a cold' becomes 'ac' (I've seen this in other clues**).
'mushroom' becomes 'cep' (Boletus edulis - an edible mushroom).
'and' means one lot of letters go next to another.
'starter' says to take the initial letters.
The initial letter of 'tomato' is 't'.
'ac'+'cep'+'t'='ACCEPT'
(Other definitions for accept that I've seen before include "Welcome" , "Agree or be willing to take" , "Take on (responsibility)" , "Say yes > to trust" , "Receive something proffered" .)