Keen person to agree to train animal (5,6)
I believe the answer is:
eager beaver
'keen person' is the definition.
(I've seen this before)
'agree to train animal' is the wordplay.
'to train' indicates an anagram (I've seen 'trained' mean this (to train can mean to grow something into a shape)).
'animal' becomes 'beaver' (beaver is a kind of animal).
'agree' with letters rearranged gives 'eager'.
'eager'+'beaver'='EAGER BEAVER'
'to' acts as a link.
(Other definitions for eager beaver that I've seen before include "Alert and active type?" , "Keen person" , "Very keen individual" , "Diligent worker" , "keen customer" .)