Serious article penned by Shackleton, perhaps (7)
I believe the answer is:
earnest
'serious' is the definition.
(similar in meaning)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for earnest that I've seen before include "Firm" , "Showing sincere and intense conviction" , "art making money?" , "Serious; down-payment" , "Grave" .)