Ace abandons struggling Boston team? It's a grave sign! (9)
I believe the answer is:
tombstone
'ace abandons struggling boston team? it's a' is the definition.
I can't tell whether this defines the answer.
'grave sign' is the wordplay.
'grave' becomes 'tomb' (synonyms).
'sign' becomes 'stone' (I can't justify this - if you can you should give a lot more credence to this answer).
'tomb'+'stone'='TOMBSTONE'
Can you help me to learn more?
(Other definitions for tombstone that I've seen before include "Graveyard marker" , "Memorial in churchyard - town in Arizona" , "Does it display departure date" , "film location" , "Last monument" .)