It's a mere triviality, but remove the shadow (6)
I believe the answer is:
detail
'it's a mere triviality' is the definition.
I can't judge whether this definition defines the answer.
'remove the shadow' is the wordplay.
'remove' becomes 'de' (I've seen this in other clues).
'the shadow' becomes 'tail' (both can mean to follow someone).
'de'+'tail'='DETAIL'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for detail that I've seen before include "Special military group" , "Tailed into particulars" , "analysts may go into" , "Relate minutely" , "A piece of information, may be minor" .)