Something in chest to check when entertaining wild run (7)
I believe the answer is:
sternum
'something in chest' is the definition.
'check when entertaining wild run' is the wordplay.
'check' becomes 'stem' (I've seen this before**).
'when entertaining' is an insertion indicator.
'wild' indicates an anagram.
'run' is an anagram of 'rnu'.
'stem' placed around 'rnu' is 'STERNUM'.
'to' is the link.
(Other definitions for sternum that I've seen before include "Chest bone" , "Breastbone" , "Human bone -- ten rums (anag)" , "somewhere in chest" , "front of breast" .)