Doctor has left to become a cattleman (6)
I believe the answer is:
drover
'a cattleman' is the definition.
(I've seen this in another clue)
'doctor has left' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'has' says to put letters next to each other.
'left' becomes 'over' (I've seen this before).
'dr'+'over'='DROVER'
'to become' acts as a link.
(Other definitions for drover that I've seen before include "Hugh the --, opera by Vaughan Williams" , "One moving a herd or flock" , "Person guiding animals to market" , "One driving flock to market" , "Herdsman" .)