Writers I've assumed to be serious (7)
I believe the answer is:
pensive
'serious' is the definition.
(thesaurus)
'writers i've' is the wordplay.
'writers' becomes 'pens' (writing implements).
'pens'+'ive'='PENSIVE'
'assumed to be' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for pensive that I've seen before include "Deeply thoughtful" , "musing" , "Engaged in deep thought" , "Introspective, maybe moody" , "Preoccupied" .)