Means to get on, heading for career in US city (6)
I believe the answer is:
agency
'means' is the definition.
(I've seen this before)
'get on heading for career in us city' is the wordplay.
'get on' becomes 'age' ('age' can be a synonym of 'get on').
'heading for' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'us city' becomes 'ny' (I've seen this in other clues**).
The first letter of 'career' is 'c'.
'c' placed into 'ny' is 'ncy'.
'age'+'ncy'='AGENCY'
'to' acts as a link.
(Other definitions for agency that I've seen before include "Type of business" , "Business that acts for another business" , "Business providing specific service" , "Recruitment bureau" , "The A in CIA" .)