In language, e.g. writing in past tense (5)
I believe the answer is:
argot
'in language e g' is the definition.
I can't judge whether this definition defines the answer.
'writing in past tense' is the wordplay.
'writing' becomes 'r' (one of the traditional three R's of education).
'in' means one lot of letters goes inside another.
'past' becomes 'ago' (similar in meaning).
'tense' becomes 't' (abbreviation used in many dictionaries).
'r' going inside 'ago' is 'argo'.
'argo'+'t'='ARGOT'
Can you help me to learn more?
(Other definitions for argot that I've seen before include "Language of a particular group" , "Cant" , "Specialised language" , "Kind of slang" , "Slang, patois" .)