Nonchalant captain started abusing heartless umpire (8)
I believe the answer is:
carefree
'nonchalant' is the definition.
(thesaurus)
'abusing heartless umpire' is the wordplay.
'abusing' becomes 'ca' (I can't justify this - if you can you should believe this answer much more).
'heartless' says to hollow out the word (remove centre letters).
'umpire' becomes 'referee' (synonyms).
'referee' with its centre taken out is 'refree'.
'ca'+'refree'='CAREFREE'
'captain started' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for carefree that I've seen before include "not concerned?" , "Untroubled" , "relaxed" , "Without worry or responsibility" , "Not bothered" .)