Be concealing the heart irregularity, or murmur (7)
I believe the answer is:
breathe
'murmur' is the definition.
(thesaurus)
'be concealing the heart irregularity' is the wordplay.
'concealing' indicates putting letters inside.
'irregularity' indicates an anagram.
'heart' with letters rearranged gives 'reath'.
'be' placed around 'reath' is 'BREATHE'.
'or' acts as a link.
(Other definitions for breathe that I've seen before include "Express" , "Take in 30" , "Whisper" , "Inhale/exhale" , "Do this before you go under" .)