Nearly everyone puttin' on show at Christmas (7)
I believe the answer is:
aladdin
'show at christmas' is the definition.
'aladdin' can be an answer for 'show' (traditional pantomime). I'm not certain of the 'at christmas' bit.
'nearly everyone puttin' on' is the wordplay.
'nearly' means to remove the last letter.
'everyone' becomes 'all' (all people).
'puttin'' becomes 'addin' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'all' with its final letter removed is 'al'.
'al'+'addin'='ALADDIN'
Can you help me to learn more?
(Other definitions for aladdin that I've seen before include "And dial up the owner of a magic lamp" , "12" , "Magic lamp rubber?" , "Widow Twankey's principal boy" , "PANTO (part title)" .)