Underground worker sounds unimportant (5)
I believe the answer is:
miner
'underground worker' is the definition.
(someone working underground in a mine)
'sounds unimportant' is the wordplay.
'sounds' shows a homophone (sound like).
'unimportant' becomes 'minor' (I've seen this in another clue).
'minor' sounds like 'MINER'.
(Other definitions for miner that I've seen before include "Worker below ground" , "One works underground" , "Digger for gold or coal, say" , "One sent down for work" , "One who digs for ore" .)