Young person, one who might work underground, would you say? (5)
I believe the answer is:
minor
'young person' is the definition.
(I've seen this before)
'one who might work underground would you say?' is the wordplay.
'one who might work underground' becomes 'miner'.
'would you say?' indicates a 'sounds like' (homophone) clue.
'miner' sounds like 'MINOR'.
(Other definitions for minor that I've seen before include "One not yet of legal age" , "Person not yet of legal age" , "Unimportant or unable to vote" , "Paltry" , "Insignificant" .)