Sound like a hack's nearly swallowing pill (5)
I believe the answer is:
neigh
'sound' is the definition.
(neigh is a kind of sound)
'nearly swallowing pill' is the wordplay.
'nearly' becomes 'nigh' (synonyms).
'swallowing' is an insertion indicator.
'pill' becomes 'e' (ecstasy).
'nigh' enclosing 'e' is 'NEIGH'.
'like a hack's' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for neigh that I've seen before include "Cry coming from mount" , "Sound coming from Ben Nevis" , "Farmyard sound" , "Comment from Arab" , "Mare's cry, e.g." .)