American faction losing weight exercising (5)
I believe the answer is:
using
'exercising' is the definition.
(I've seen this before)
'american faction losing weight' is the wordplay.
'american' becomes 'us'.
'faction' becomes 'wing'.
'losing' is a deletion indicator.
'weight' becomes 'w'.
'wing' with 'w' taken away is 'ing'.
'us'+'ing'='USING'
(Other definitions for using that I've seen before include "addiction" , "practice" , "Putting into service" , "Exercising" , "Employing" .)