No outspoken nag's comment (5)
I believe the answer is:
neigh
'nag's comment' is the definition.
(sound made by a horse)
'no outspoken' is the wordplay.
'no' becomes 'nay' (archaic form of 'no').
'outspoken' indicates a 'sounds like' (homophone) clue (as the word might be spoken).
'nay' sounds like 'NEIGH'.
(Other definitions for neigh that I've seen before include "one made by Bucephalus?" , "Whinnying noise" , "nicker" , "Hinge (anag.)" , "High-pitched cry of a horse" .)