Outspoken verse on developing coal (5)
I believe the answer is:
vocal
'outspoken' is the definition.
(synonyms)
'verse on developing coal' is the wordplay.
'verse' becomes 'v' ().
'on' indicates putting letters inside.
'developing' indicates anagramming the letters (letters developed into a new form).
'coal' anagrammed gives 'ocal'.
'v' going within 'ocal' is 'VOCAL'.
(Other definitions for vocal that I've seen before include "not instrumental" , "Loud in complaining" , "Given to speech, often in protest" , "Audible in protest" , "Articulate" .)