Speed of agency typist with a circular letter (5)
I believe the answer is:
tempo
'speed' is the definition.
(thesaurus)
'agency typist with a circular letter' is the wordplay.
'agency typist' becomes 'temp' (I can't explain this - if you can you should believe this answer much more).
'with' is a charade indicator (letters next to each other).
'a circular letter' becomes 'o'.
'temp'+'o'='TEMPO'
'of' is the link.
Can you help me to learn more?
(Other definitions for tempo that I've seen before include "Muscial time" , "Rhythm, metre" , "Speed or beat, musically" , "musical quality?" , "Time (music)" .)