Tomboy initially trains dog (5)
I believe the answer is:
trail
'dog' is the definition.
(both can mean to follow)
'tomboy initially trains' is the wordplay.
'initially' suggests taking the first letters.
'trains' becomes 'rail' (I've seen this in another clue).
The initial letter of 'tomboy' is 't'.
't'+'rail'='TRAIL'
(Other definitions for trail that I've seen before include "Scent or track followed in hunting" , "Rural footpath" , "Need to catch up with" , "Forest path" , "Beaten path; track" .)