Sense brief opening — sign on staff (5,4)
I believe the answer is:
tenor clef
'sign on staff' is the definition.
(sign on a musical staff)
'sense brief opening' is the wordplay.
'sense' becomes 'tenor' (both can mean something's gist or meaning).
'brief' means to remove the last letter (a shorter or briefer version of the word).
'opening' becomes 'cleft' (cleft is a kind of opening).
'cleft' with its final letter taken away is 'clef'.
'tenor'+'clef'='TENOR-CLEF'
(Other definitions for tenor clef that I've seen before include "Character instructing musician" , "One on the staff" , "instruction for a musician" , "It has middle C on second- highest line" , "A stave marking for pitch" .)