Sound but undersized specimen with a pig’s tail (5)
I believe the answer is:
grunt
'sound' is the definition.
(I have seen 'Inarticulate sound ' mean 'grunt' so perhaps 'sound' could also mean 'grunt')
'undersized specimen with a pig's tail' is the wordplay.
'undersized specimen' becomes 'runt' (I am not sure about this - if you are sure you should believe this answer much more).
'with' is a charade indicator (letters next to each other).
'tail' says to take the final letters.
The final letter of 'pig' is 'g'.
'runt' put after 'g' is 'GRUNT'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for grunt that I've seen before include "Beastly sound" , "Talk like a pig" , "Noise of a pig ..." , "Snort (like a pig)" , "Sound made by a pig" .)