Write all day on a regular basis? That's manageable (6)
I believe the answer is:
wieldy
'that's manageable' is the definition.
(synonyms)
'write all day on a regular basis?' is the wordplay.
'on a regular basis?' indicates alternate letters.
'write'+'all'+'day'='writeallday'
The alternating letters of 'writeallday' are 'WIELDY'.
(Other definitions for wieldy that I've seen before include "Easy to control" , "Manageable" , "Easy to manage" .)