It's easy killing one immobile deer (1,4,4)
I believe the answer is:
a fast buck
'it's easy killing' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'one immobile deer' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is found within the answer.
'immobile' could be 'fast' (similar in meaning) and 'fast' is present in the answer.
'deer' could be 'buck' (male deer) and 'buck' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for a fast buck that I've seen before include "Easy money" , "easily made money" , "One wild young fellow" , "Does find it difficult to go along with this" , "killing?" .)