One who once lived a hollow life (7)
I believe the answer is:
caveman
'one who once lived a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'hollow life' is the wordplay.
'hollow' becomes 'cave' (caving is a kind of hollowing).
'life' becomes 'man' (I can't explain this - if you can you should believe this answer much more).
'cave'+'man'='CAVEMAN'
Can you help me to learn more?
(Other definitions for caveman that I've seen before include "Prehistoric person" , "Early hominid" , "he has primitive habits" , "Prehistoric male" , "Van came carrying old cliff dweller" .)