Speed means nothing to a casual worker (5)
I believe the answer is:
tempo
'speed' is the definition.
(thesaurus)
'nothing to a casual worker' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'to a' says to put letters next to each other (I've seen this in other clues).
'casual worker' becomes 'temp' (I've seen this before**).
'o' put after 'temp' is 'TEMPO'.
'means' acts as a link.
(Other definitions for tempo that I've seen before include "Musical beat or rhythm" , "Speed or beat, musically" , "Rhythm or speed as in music" , "Speed of 3A" , "Speed of a musical piece" .)