For speed, use them in top, perhaps (5)
I believe the answer is:
tempo
'for speed' is the definition.
(thesaurus)
'them in top perhaps' is the wordplay.
'them' becomes 'em'.
'in' is an insertion indicator.
'perhaps' is an anagram indicator.
'top' anagrammed gives 'tpo'.
'em' put within 'tpo' is 'TEMPO'.
'use' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tempo that I've seen before include "Pace of a piece of music" , "Speed at which a musical composition is to be played" , "Musical time" , "Pace - pacing" , "Rhythm, metre" .)