Most significant sources of inspiration clearly matter (5)
I believe the answer is:
topic
'matter' is the definition.
('topic' can be a synonym of 'matter')
'most significant sources of inspiration clearly' is the wordplay.
'most significant' becomes 'top'.
'sources of' indicates taking the first letters.
The initial letters of 'inspiration clearly' is 'ic'.
'top'+'ic'='TOPIC'
(Other definitions for topic that I've seen before include "Content of argument" , "Subject (for conversation, etc)" , "Theme (of, eg, a talk)" , "might this be hot?" , "Matter for an argument" .)