Joint endlessly screening musical up in bar (8)
I believe the answer is:
obstacle
'bar' is the definition.
(thesaurus)
'joint endlessly screening musical up in' is the wordplay.
'joint' becomes 'obl' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'endlessly' means to look at the middle letters (the ends of the words are removed).
'musical' becomes 'cats' (I've seen this before).
'up' is a reversal indicator (in down clue: letters go upwards).
'in' means one lot of letters goes inside another.
The centre of 'screening' is 'e'.
'cats' backwards is 'stac'.
'obl'+'e'='oble'
'oble' placed around 'stac' is 'OBSTACLE'.
Can you help me to learn more?
(Other definitions for obstacle that I've seen before include "It obstructs or hinders" , "stumbling block" , "Barrier or deterrent" , "Hindrance, obstruction" , "bar" .)