Fool endlessly gaining marks for natural expression (5)
I believe the answer is:
idiom
'natural expression' is the definition.
'idiom' can be an answer for 'expression' (idiom is a kind of expression). I'm unsure of the 'natural' bit.
'fool endlessly gaining marks' is the wordplay.
'fool' becomes 'idiot' (synonyms).
'endlessly' means to remove the last letter.
'gaining marks' becomes 'm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'idiot' with its final letter taken off is 'idio'.
'idio'+'m'='IDIOM'
'for' is the link.
Can you help me to learn more?
(Other definitions for idiom that I've seen before include "Figure of speech" , "Phrase such as over the moon" , "Dialect, vernacular" , "Distinctive expression" , "Distinctive speech" .)