Large wildebeest rolls over to get a breather (4)
I believe the answer is:
lung
'a breather' is the definition.
(I've seen this before)
'large wildebeest rolls over' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'wildebeest' becomes 'gnu' (synonyms).
'rolls over' is a reversal indicator (I've seen 'rolling over' mean this).
'gnu' in reverse letter order is 'ung'.
'l'+'ung'='LUNG'
'to get' is the link.
(Other definitions for lung that I've seen before include "One of a pair of organs in the ribcage" , "Pulmonary organ" , "breather" , "Breathing apparatus" , "Organ providing oxygen to the blood" .)