The ultimate in light weight lifting (5)
I believe the answer is:
theft
'lifting' is the definition.
(I've seen this before)
'the ultimate in light weight' is the wordplay.
'the ultimate in' suggests the final letters (take the ultimate ie last letter).
'weight' becomes 'heft' (heft is a kind of weight).
The final letter of 'light' is 't'.
't'+'heft'='THEFT'
(Other definitions for theft that I've seen before include "Wrongly taking" , "Taking of another's property" , "Misappropriation" , "Act of larceny" , "Taking offence?" .)