Mechanical routine giving balls energy (4)
I believe the answer is:
rote
'mechanical routine' is the definition.
(as in learning by rote)
'balls energy' is the wordplay.
'balls' becomes 'rot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'energy' becomes 'e' (physics symbol as in E=mco).
'rot'+'e'='ROTE'
'giving' acts as a link.
Can you help me to learn more?
(Other definitions for rote that I've seen before include "Kind of memory - that tore it" , "mindless activity?" , "By . . . . - by memory without thinking" , "performance based solely on routine" , "Way of learning" .)