Serious scowl key man regularly produces (6)
I believe the answer is:
solemn
'serious' is the definition.
(similar in meaning)
'scowl key man regularly produces' is the wordplay.
I cannot quite see how this works, but
'key' could be 'e' (musical key) and 'e' is present in the answer.
'man' could be 'sol' and 'sol' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for solemn that I've seen before include "'Earnest, grave (6)'" , "'Dignified, sombre (6)'" , "'Sedate, grave (6)'" , "Serious; deeply sincere" , "Melons (anag.)" .)