Take mushrooms, put together with dash of eggnog and offal (5)
I believe the answer is:
tripe
'offal' is the definition.
(I've seen this before)
'take mushrooms put together with dash of eggnog' is the wordplay.
'take mushrooms' becomes 'trip' (as in a drugs trip).
'put together with' means one lot of letters go next to another.
'dash of' indicates taking the first letters.
The first letter of 'eggnog' is 'e'.
'trip'+'e'='TRIPE'
'and' acts as a link.
(Other definitions for tripe that I've seen before include "Ox stomach as food - it's nonsense" , "Edible lining of cow's stomach - rubbish" , "Rubbish, nonsense" , "Rhubarb" , "Ruminant's stomach lining" .)