Be way out keeping score, losing heart (5)
I believe the answer is:
exist
'be' is the definition.
(I know that be can be written as exist)
'way out keeping score losing heart' is the wordplay.
'way out' becomes 'exit' (synonyms).
'keeping' is an insertion indicator.
'score losing heart' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'exit' enclosing 's' is 'EXIST'.
Can you help me to learn more?
(Other definitions for exist that I've seen before include "Get a life!" , "Be alive" , "Survive" , "Live" , "Scrape by" .)