Complete fool when forgetting name (5)
I believe the answer is:
utter
'complete' is the definition.
(I know that complete can be written as utter)
'fool when forgetting name' is the wordplay.
'fool' becomes 'nutter' (both can mean an eccentric person).
'when forgetting' is a deletion indicator.
'name' becomes 'n' (common abbreviation e.g. on forms).
'nutter' with 'n' removed is 'UTTER'.
(Other definitions for utter that I've seen before include "Speak; completely" , "Absolute - say" , "Make (a sound); complete" , "Positive" , "Say - complete" .)