Transport engineers first to prepare for new job? (7)
I believe the answer is:
retrain
'prepare for new job?' is the definition.
(train for a new career)
'transport engineers first' is the wordplay.
'transport' becomes 'train' (train is a kind of transport).
'engineers' becomes 're' (abbreviation for Royal Engineers).
'first' is a charade indicator (letters next to each other).
'train' after 're' is 'RETRAIN'.
'to' is the link.
(Other definitions for retrain that I've seen before include "Terrain (anag.)" , "Teach (someone) new skills" , "in school again" , "Study for new job" , "Take a different course" .)