To put a fellow in the shade is perfectly decent (6)
I believe the answer is:
humane
'perfectly decent' is the definition.
'humane' can be an answer for 'decent' (thesaurus). I'm not sure about the 'perfectly' bit.
'to put a fellow in the shade' is the wordplay.
'to put' means one lot of letters goes inside another.
'a' becomes 'an'.
'fellow in the shade' becomes 'hume' (I am not sure about this - if you are sure you should believe this answer much more).
'an' placed into 'hume' is 'HUMANE'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for humane that I've seen before include "Characterised by kindness" , "Showing compassion or consideration for others" , "Benign - kind" , "Kind and merciful" , "sort of killer" .)