A piece of cake from party to go off without lid (6)
I believe the answer is:
doddle
'a piece of cake' is the definition.
(both can mean something very easy)
'party to go off without lid' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'to go off' becomes 'addle' (eg an egg might addle).
'without lid' means to remove the first letter.
'addle' with its initial letter taken away is 'ddle'.
'do'+'ddle'='DODDLE'
'from' is the link.
(Other definitions for doddle that I've seen before include "Absurdly simple" , "Snip" , "pushover" , "Piece of cake" , "In slang, an easy task" .)