Just for a bit of lunch, a chap's not dressed (6)
I believe the answer is:
unclad
'just for a bit of lunch a chap's' is the definition.
I can't tell whether this definition defines the answer.
'not dressed' is the wordplay.
'not' becomes 'un' (I've seen this in other clues).
'dressed' becomes 'clad' (to be clad in something is to be dressed in it).
'un'+'clad'='UNCLAD'
Can you help me to learn more?
(Other definitions for unclad that I've seen before include "having abandoned habit" , "In one's birthday suit" , "Not wearing clothes" , "with nothing on" , "nude" .)