Reproduction of giant mice is not simple (9)
I believe the answer is:
enigmatic
'not simple' is the definition.
(enigmatic can mean complex or confusing)
'reproduction of giant mice' is the wordplay.
'reproduction of' is an anagram indicator (the same letters produced differently).
'giant'+'mice'='giantmice'
'giantmice' is an anagram of 'ENIGMATIC'.
'is' acts as a link.
(Other definitions for enigmatic that I've seen before include "Hard to fathom" , "with mysterious substance" , "with a mystery ingredient" , "Difficult to interpret" , "Mysterious or puzzling, like Mona Lisa's smile" .)