Unit of force, current and weight (6)
I believe the answer is:
newton
'unit of force' is the definition.
(I've seen this before)
'current and weight' is the wordplay.
'current' becomes 'new' ('new' is associated in meaning with 'current').
'and' means one lot of letters go next to another.
'weight' becomes 'ton' (ton is a kind of weight).
'new'+'ton'='NEWTON'
(Other definitions for newton that I've seen before include "inclined to apply discovery?" , "Sir Isaac --; -- Abbot" , "'English physicist, described the law of gravity (6)'" , "Gravity theorist" , "Scientist - unit of force" .)