First person heard clean nonsense (7)
I believe the answer is:
eyewash
'nonsense' is the definition.
(I've seen this before)
'first person heard clean' is the wordplay.
'first person' becomes 'I' (first-person pronoun).
'heard' indicates a 'sounds like' (homophone) clue.
'clean' becomes 'wash' (washing is a kind of cleaning).
'i' sounds like 'eye'.
'eye'+'wash'='EYEWASH'
(Other definitions for eyewash that I've seen before include "Optic solution to relieve irritation - rubbish!" , "Nonsense; cleansing lotion" , "Insincere talk or optical lotion" , "Visual cleansing agent - nonsense" , "Pupils may benefit from this" .)