Paul abandons population by choice (6)
I believe the answer is:
option
'choice' is the definition.
(I know that choice can be written as option)
'paul abandons population' is the wordplay.
'abandons' is a deletion indicator.
'population' with 'paul' taken out is 'OPTION'.
'by' acts as a link.
(Other definitions for option that I've seen before include "Something choosable" , "possibility" , "Thing that may be chosen" , "Possible choice" , "Alternative for choice" .)