A journey interrupted by massive heart attack (6)
I believe the answer is:
assail
'attack' is the definition.
(synonyms)
'a journey interrupted by massive heart' is the wordplay.
'journey' becomes 'sail' (sailing is a kind of journeying).
'interrupted by' indicates putting letters inside.
'heart' means to look at the middle letters.
The central letter of 'massive' is 's'.
'sail' enclosing 's' is 'ssail'.
'a'+'ssail'='ASSAIL'
(Other definitions for assail that I've seen before include "Assault, attack" , "Criticize" , "Molest or harass" , "storm" , "Attack violently and persistently" .)