Letters kept by fussy MP to make mark (7)
I believe the answer is:
symptom
'letters' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'kept by fussy mp to make mark' is the wordplay.
'kept by' says the answer is hidden in the clue.
'SYMPTOM' can be found hidden inside 'fussy mp to make mark'.
Can you help me to learn more?
(Other definitions for symptom that I've seen before include "Sign of disease" , "Indicator of disease" , "Feature of a disease" , "Expression" , "Indication (of disease, problem)" .)