It checks the growth of population (6)
I believe the answer is:
census
This clue might be a double definition.
'it checks' is the first definition.
The definition suggests a singular noun which matches the answer.
'growth of population' is the second definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'the' is the link.
Can you help me to learn more?
(Other definitions for census that I've seen before include "Periodic counting of the population" , "People count" , "Population survey" , "Periodic count by statisticians" , "Official count of population" .)