I'm a moving target (3)
I believe the answer is:
'target' is the definition.
(both can mean a goal or purpose)
'i'm a moving' is the wordplay.
'moving' indicates an anagram (letters are moved around).
'ima' with letters rearranged gives 'AIM'.
(Other definitions for aim that I've seen before include "intent" , "Try to achieve" , "Point towards a target, or have ambition" , "Plan" , "Attempt" .)