Some carols to sing while watching the joint (6)
I believe the answer is:
casing
'while watching the joint' is the definition.
I can't tell whether this definition defines the answer.
'some carols to sing' is the wordplay.
'some carols' becomes 'ca' (I can't justify this - if you can you should believe this answer much more).
'ca'+'sing'='CASING'
Can you help me to learn more?
(Other definitions for casing that I've seen before include "Protective covering as of engine" , "Protective shell" , "Outer protective covering" , "Outside cover" , "Recceing" .)