Go up first, it's an improvement (6)
I believe the answer is:
upturn
'an improvement' is the definition.
(upturn is a kind of improvement)
'go up first' is the wordplay.
'go' becomes 'turn' ('turn' can be a synonym of 'go'**).
'first' means one lot of letters go next to another.
'turn' after 'up' is 'UPTURN'.
'it's' acts as a link.
(Other definitions for upturn that I've seen before include "Recovery" , "economic improvement" , "Improved trend in business or fortune" , "Boom after a bust" , "Improvement in business activity" .)