Muscle in, like many Americans (5)
I believe the answer is:
latin
'muscle in like many americans' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'muscle in' is the wordplay.
'muscle' becomes 'lat' (short for latissimus muscle).
'lat'+'in'='LATIN'
Can you help me to learn more?
(Other definitions for latin that I've seen before include "Romantic language" , "Parent language of French,Romanian" , "ancient tongue" , "A Romance language" , "Language of ancient Rome" .)