Composer's third and fifth maybe (9)
I believe the answer is:
intervals
'composer's third and fifth maybe' is the definition.
(third and fifth are types of musical interval)
This is the entire clue.
(Other definitions for intervals that I've seen before include "Times between acts" , "Intermissions" , "Net's viral (anag.)" , "Rests" , "Periods of time between parts of a play" .)