It's a beast to keep on track! (5)
I believe the answer is:
steer
'track' is the definition.
The definition and answer can be both to do with motion as well as being verbs in their base form.
Perhaps there's an association between them I don't understand?
'it's a beast to keep' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'a beast' becomes 'seer' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to keep' is an insertion indicator.
't' put inside 'seer' is 'STEER'.
'on' is the link.
Can you help me to learn more?
(Other definitions for steer that I've seen before include "Guide, control" , "Control movement" , "cow!" , "Guide - castrated ox" , "Manoeuvre" .)