A rating on a phone battery (7)
I believe the answer is:
assault
This clue could be a double definition.
'a rating' is the first definition.
(thesaurus)
'phone battery' is the second definition.
'assault' can be an answer for 'battery' (synonyms). I am not certain of the 'phone' bit.
'on a' is the link.
Can you help me to learn more?
(Other definitions for assault that I've seen before include "raid" , "Sort of course" , "Violent attack in action or words" , "set about" , "Strike" .)