The strength of the current? (8)
I believe the answer is:
amperage
This clue is a double definition.
'the strength' is the first definition.
(I have seen 'Strength of current' mean 'amperage' so perhaps 'strength' could also mean 'amperage')
'the current?' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for amperage that I've seen before include "current measure" , "Strength of current" .)