Cumbrian border - pull over and have a breather (4)
I believe the answer is:
lung
'breather' is the definition.
(I've seen this before)
'cumbrian border pull over' is the wordplay.
'cumbrian' becomes 'un' (I can't justify this - if you can you should give a lot more credence to this answer).
'border' suggests removing the centre (I've seen 'borders' mean this).
'pull' becomes 'lug' (I've seen this before).
'over' means one lot of letters goes inside another.
'lug' with its centre removed is 'lg'.
'un' put inside 'lg' is 'LUNG'.
'and have a' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lung that I've seen before include "Respiratory organ" , "breather" , "One of a pair of organs in the ribcage" , "Air bag?" , "Organ used for breathing" .)