Frenchman in America taking off (5)
I believe the answer is:
minus
'taking off' is the definition.
'frenchman in america' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'america' becomes 'US' (United States).
'm'+'in'+'us'='MINUS'
(Other definitions for minus that I've seen before include "Reduced by the subtraction of - thus?" , "Lacking; below zero" , "Negative arithmetic operation" , "'Take away' sign" , "(Of temperature) below zero" .)