For a time, silent about a measure (6)
I believe the answer is:
minute
'for a time' is the definition.
(I've seen this before)
'silent about a measure' is the wordplay.
'silent' becomes 'mute' (synonyms).
'about' is an insertion indicator.
'a measure' becomes 'in' (in is a kind of measure).
'mute' placed around 'in' is 'MINUTE'.
(Other definitions for minute that I've seen before include "Little -- 60 18 across" , "Tiny; written note" , "Extremely small" , "60 seconds; tiny" , "Brief note" .)