Weight around a ton gives a measure of power (4)
I believe the answer is:
watt
'a measure of power' is the definition.
(unit of power)
'weight around a ton' is the wordplay.
'weight' becomes 'wt'.
'around' means one lot of letters goes inside another.
'ton' becomes 't' (abbreviation).
'wt' placed around 'a' is 'wat'.
'wat'+'t'='WATT'
'gives' is the link.
(Other definitions for watt that I've seen before include "man of power?" , "James -; Sc. engineer" , "Unit of electric power" , "SI unit of power" , "Unit of electricity" .)