This sharp-witted writer seeks, only in small amount (3,5)
I believe the answer is:
mot juste
'this sharp-witted writer seeks' is the definition.
The definition suggests a singular noun which matches the answer.
'only in small amount' is the wordplay.
'only' becomes 'just' (synonyms).
'in' means one lot of letters goes inside another.
'small amount' becomes 'mote' (I am not sure about this - if you are sure you should believe this answer much more).
'just' inserted into 'mote' is 'MOT JUSTE'.
Can you help me to learn more?
(Other definitions for mot juste that I've seen before include "Exactly appropriate word" , "Highly appropriate expression" .)