American ruler loses kilo exercising (5)
I believe the answer is:
using
'exercising' is the definition.
(I've seen this before)
'american ruler' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'ruler' becomes 'ing' (I can't explain this - if you can you should give a lot more credence to this answer).
'us'+'ing'='USING'
'loses kilo' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for using that I've seen before include "practice" , "taking drugs" , "having recourse to" , "Utilising" , "Exercising" .)