Sense deer starts to get some understanding (9)
I believe the answer is:
hindsight
'some understanding' is the definition.
(hindsight is a kind of understanding)
'sense deer starts' is the wordplay.
'sense' becomes 'sight' (sight or vision is a sense).
'deer' becomes 'hind' (hind is a kind of deer).
'starts' means one lot of letters go next to another.
'sight' put after 'hind' is 'HINDSIGHT'.
'to get' is the link.
(Other definitions for hindsight that I've seen before include "Thinking back" , "Wisdom after the event" , "Ability to understand, perhaps too late" , "acquired too late to be useful?" , "review" .)