Tag one article in English gym (7)
I believe the answer is:
epithet
'tag' is the definition.
(both can mean a nickname)
'one article in english gym' is the wordplay.
'one' becomes 'i' (roman numeral).
'article' becomes 'the' (the 'definite article' part of speech).
'in' indicates putting letters inside.
'english' becomes 'e' (abbreviation).
'gym' becomes 'pt'.
'i'+'the'='ithe'
'e'+'pt'='ept'
'ithe' put into 'ept' is 'EPITHET'.
(Other definitions for epithet that I've seen before include "Fitting description" , "Adjective expressing some quality of a person" , "Descriptive adjective" , "title" , "Descriptive work or phrase" .)