Enduring heartless command (6)
I believe the answer is:
biding
'enduring' is the definition.
The definition and answer can be both to do with fundamental existence as well as being present participle verbs.
Perhaps there's a link between them I don't understand?
'heartless command' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'command' becomes 'bidding' (synonyms).
'bidding' with its centre taken out is 'BIDING'.
Can you help me to learn more?
(Another definition for biding that I've seen is " Remaining in a place".)