Keen writer's clear about source of inspiration (9)
I believe the answer is:
impatient
'keen' is the definition.
(thesaurus)
'writer's clear about source of inspiration' is the wordplay.
'writer's' becomes 'im' ('writer's' could mean 'the crossword setter is').
'clear' becomes 'patent' ('patent' can be a synonym of 'clear'**).
'about' indicates putting letters inside.
'source of' indicates taking the first letters.
The initial letter of 'inspiration' is 'i'.
'patent' enclosing 'i' is 'patient'.
'im'+'patient'='IMPATIENT'
(Other definitions for impatient that I've seen before include "showing anxiety" , "those who hate waiting" , "Restless; intolerant" , "Over-eager" , "Snappy" .)