Glow bug by rocks starts to combust eventually (9)
I believe the answer is:
fluoresce
'glow' is the definition.
(fluorescing is a kind of glowing)
'bug by rocks starts to combust eventually' is the wordplay.
'bug' becomes 'flu' (influenza).
'by' says to put letters next to each other.
'rocks' becomes 'ores' (I've seen this before).
'starts to' suggests taking the first letters.
The first letters of 'combust eventually' is 'ce'.
'flu'+'ores'+'ce'='FLUORESCE'
(Other definitions for fluoresce that I've seen before include "Emit light when irradiated" , "glow brightly" .)