Build up internal resistance for attack (6)
I believe the answer is:
stroke
'attack' is the definition.
(I know that stroke is a more specific form of the action hit)
'build up internal resistance for' is the wordplay.
I cannot quite understand how this works, but
'resistance' could be 'r' (electronics abbreviation) and 'r' is found within the answer.
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is located in the leftover letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for stroke that I've seen before include "beat?" , "Fit of apoplexy" , "Rub gently" , "one of rowing crew" , "Blow - gentle touch - brain damage" .)