Boy to prepare for journey with intermittent steroid injection (7)
I believe the answer is:
patrick
'boy' is the definition.
'prepare for journey with intermittent steroid injection' is the wordplay.
'prepare for journey' becomes 'pack' (eg pack a suitcase).
'with' means one lot of letters goes inside another.
'intermittent steroid injection' becomes 'tri' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pack' enclosing 'tri' is 'PATRICK'.
'to' acts as a link.
(Other definitions for patrick that I've seen before include "Ireland's patron saint" , "Patron saint who said 'Shoo!' to serpents" , "-- Moore, astronomer" , "- - Swayze: film actor" , "Patron saint of Ireland" .)