To use extravagant praise, lay it on this (5)
I believe the answer is:
thick
'to use extravagant praise lay it on this' is the definition.
The definition suggests a singular noun which matches the answer.
'it on this' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'on this' becomes 'hick' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
't'+'hick'='THICK'
Can you help me to learn more?
(Other definitions for thick that I've seen before include "Broad; dense" , "Wide -- impenetrable" , "Wide, broad" , "Friendly" , "unintelligent?" .)