A man indisposed to enter flat backward (7)
I believe the answer is:
neville
'a man' is the definition.
'indisposed to enter flat backward' is the wordplay.
'indisposed' becomes 'ill' (similar in meaning).
'to enter' indicates putting letters inside.
'flat' becomes 'even' (similar in meaning).
'backward' says the letters should be written in reverse.
'even' backwards is 'neve'.
'ill' put into 'neve' is 'NEVILLE'.
(Other definitions for neville that I've seen before include "Richard --, Earl of Warwick" , "N Chamberlain, PM" , "- Longbottom, friend of Harry Potter" .)