From birth to death (8)
(Other definitions for lifelong that I've seen before include "Lasting from birth to death" , "serious custodial sentence" , "Till death" , "Birth-to-death" , "with no possibility of release" .)
'from birth to death' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lifetime that I've seen before include "Years" , "Person's duration" , "From birth to death" , "The seven ages of man" , "Seemingly an age?" .)