Number set about making the interior cheerful (7)
I believe the answer is:
festive
'cheerful' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?
'number set about making the interior' is the wordplay.
'number' becomes 'five' (five is a number).
'set about making' becomes 'est' (I can't justify this - if you can you should give a lot more credence to this answer).
'the interior' means one lot of letters goes inside another.
'five' going around 'est' is 'FESTIVE'.
Can you help me to learn more?
(Other definitions for festive that I've seen before include "Celebratory, joyful" , "Jolly" , "Merry" , "Celebrating" , "Offering fun and gaiety" .)