Assume nothing about a Lord Fauntleroy? (6,4)
I believe the answer is:
donald duck
'fauntleroy?' is the definition.
(Fauntleroy is Donald Duck's middle name)
'assume nothing about a lord' is the wordplay.
'assume' becomes 'don' (I've seen this before).
'nothing' becomes 'duck' (duck means zero in cricket).
'about' means one lot of letters go next to another.
'lord' becomes 'ld' (abbreviation for the aristocratic title).
'a'+'ld'='ald'
'duck' after 'ald' is 'aldduck'.
'don'+'aldduck'='DONALD DUCK'
(Other definitions for donald duck that I've seen before include "Bradman getting no runs" , "would be a figure of fun" , "Disney cartoon character" , "Screen favourite" , "Walt Disney character" .)