Son! Councillor is starting exercise for writing! (6)
I believe the answer is:
script
'writing' is the definition.
(script is a kind of writing)
'son councillor is starting exercise' is the wordplay.
'son' becomes 's'.
'councillor' becomes 'cr' (I've seen this in other clues).
'starting' indicates taking the first letters.
'exercise' becomes 'pt'.
The first letter of 'is' is 'i'.
's'+'cr'+'i'+'pt'='SCRIPT'
'for' is the link.
(Other definitions for script that I've seen before include "Orginal document" , "Text of play, film etc" , "Performance text" , "Calligraphy" , "Words of film etc" .)