Mistake slime for yellow leaves (4)
I believe the answer is:
goof
'mistake' is the definition.
(I've seen this before)
'slime for yellow leaves' is the wordplay.
'slime' becomes 'goo' (synonyms).
'yellow' becomes 'or' (heraldic term).
'leaves' suggests deleting specific letters.
'for' with 'or' taken away is 'f'.
'goo'+'f'='GOOF'
(Other definitions for goof that I've seen before include "(Make a) silly mistake" , "boob" , "Foolish mistake" , "In slang, bungle or blunder" , "Blunder, mistake (slang)" .)