Without them, could be top speed (5)
I believe the answer is:
tempo
'speed' is the definition.
(thesaurus)
'without them could be top' is the wordplay.
'without' means one lot of letters goes inside another.
'them' becomes 'em'.
'could be' indicates anagramming the letters.
'top' is an anagram of 'tpo'.
'em' placed inside 'tpo' is 'TEMPO'.
(Other definitions for tempo that I've seen before include "Cadence" , "Musical beat or rhythm" , "Speed of 3A" , "Rhythm or speed as in music" , "Rate and rhythm in music" .)