Actually not less than a stone (3)
I believe the answer is:
ton
'actually not' is the definition.
I can't tell whether this definition defines the answer.
'less than a stone' is the wordplay.
'less' indicates a hidden word (the clue less some of the letters in it).
'TON' is hidden within 'than stone'.
Can you help me to learn more?
(Other definitions for ton that I've seen before include "100 mph, runs, etc" , "Unit of weight equivalent to 20 cwts" , "Imperial unit of weight" , "A slangy 100 mph - heavy" , "2,240 lbs" .)