Limitless, old writer concluded (4,5)
I believe the answer is:
open ended
'limitless' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
'old writer concluded' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'writer' becomes 'pen' (something which writes).
'concluded' becomes 'ended' (synonyms).
'o'+'pen'+'ended'='OPEN-ENDED'
Can you help me to learn more?
(Other definitions for open ended that I've seen before include "Unlimited" , "not rigorously fixed" , "adaptable" , "without fixed deadlines" , "having no fixed limits" .)