US poet improved briefly in paper (5)
I believe the answer is:
frost
'us poet' is the definition.
(20th-century poet Robert Frost)
'improved briefly in paper' is the wordplay.
'improved' becomes 'rose' (rise can mean to improve or go up).
'briefly' means to remove the last letter.
'in' indicates putting letters inside.
'paper' becomes 'FT' (Financial Times newspaper).
'rose' with its final letter taken off is 'ros'.
'ros' placed within 'ft' is 'FROST'.
(Other definitions for frost that I've seen before include "Chilly feeling" , "Icy condensation" , "Robert ---, US poet" , "Rime" , "Icy deposit" .)