… only briefly, note (3)
I believe the answer is:
sol
'note' is the definition.
(I've seen this before)
'only briefly' is the wordplay.
'only' becomes 'sole' ('sole' can be a synonym of 'only').
'briefly' means to remove the last letter.
'sole' with its last letter taken off is 'SOL'.
(Other definitions for sol that I've seen before include "Roman counterpart of Helios" , "Peruvian currency; Roman sun god" , "Spanish and poetic sun" , "The sun in Spain" , "The sun in 12 Across" .)