One marks (twice) poems ending in sentiment? Shameless! (8)
I believe the answer is:
immodest
'shameless' is the definition.
(I've seen this before)
'one marks twice poems ending in sentiment?' is the wordplay.
'one' becomes 'i' (roman numeral).
'marks' becomes 'm' (former German currency).
'twice' indicates the doubling of some letters.
'poems' becomes 'odes' (ode is a kind of poem).
'ending in' says to take the final letters.
The last letter of 'sentiment' is 't'.
'm' duplicated is 'mm'.
'i'+'mm'+'odes'+'t'='IMMODEST'
(Other definitions for immodest that I've seen before include "bold" , "blowing its own trumpet" , "Swift proposal was not" , "Cheeky" , "Brazen" .)