Idiot forgetting his name is mud? (3)
I believe the answer is:
goo
'is mud?' is the definition.
(I've seen this in another clue)
'idiot forgetting his name' is the wordplay.
'idiot' becomes 'goon' (goon can mean an idiotic person).
'forgetting his' indicates named letters should be taken away.
'name' becomes 'n' (common abbreviation e.g. on forms).
'goon' with 'n' taken away is 'GOO'.
(Other definitions for goo that I've seen before include "syrup" , "Sticky substance (sl.)" , "Anything sticky" , "Sticky wet material or sickly sentiment" , "Sticky or slimy substance" .)