It enriches, making a man timelessly secure (6)
I believe the answer is:
manure
'it enriches' is the definition.
The definition suggests a singular noun which matches the answer.
'a man timelessly secure' is the wordplay.
'man' becomes 'N' (knight, a man in chess).
'timelessly' becomes 'mure' (I can't justify this - if you can you should give a lot more credence to this answer).
'secure' indicates putting letters inside (inserted letters are secured inside).
'a'+'n'='an'
'an' going into 'mure' is 'MANURE'.
'making' is the link.
Can you help me to learn more?
(Other definitions for manure that I've seen before include "Dung as fertiliser" , "Animal excreta used to fertilise land" , "Fertilizing dung" , "Animal excreta as fertiliser" , "Natural animal fertiliser" .)