Spinner's round pitch, backing up (7)
I believe the answer is:
diabolo
'spinner's round pitch backing up' is the definition.
I can't tell whether this definition defines the answer.
'spinner's round pitch backing up' is the wordplay.
'spinner's round' becomes 'o' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pitch' becomes 'lob' (both can mean to throw).
'backing' becomes 'aid' (I've seen this in another clue).
'up' is a reversal indicator (in down clue: letters go upwards).
'o'+'lob'+'aid'='olobaid'
'olobaid' back-to-front is 'DIABOLO'.
Can you help me to learn more?
(Other definitions for diabolo that I've seen before include "Old game" , "Devilish" , "Game involving sticks and spinning top" , "juggling performance?" , "Top-on-a-string game" .)