Masquerade as Dr Seuss briefly playing piano (5,2)
I believe the answer is:
dress up
'masquerade' is the definition.
(I've seen this before)
'dr seuss briefly playing piano' is the wordplay.
'briefly' means to remove the last letter.
'playing' indicates an anagram.
'piano' becomes 'p'.
'seuss' with its last letter taken away is 'seus'.
'seus' with letters rearranged gives 'essu'.
'dr'+'essu'+'p'='DRESS UP'
'as' is the link.
(Other definitions for dress up that I've seen before include "Adopt costume" , "Put on your most elegant clothes" , "Make (something) more interesting" , "Put on the glad rags" , "What spin doctors do" .)