Fleece that is for beginner (6)
I believe the answer is:
'beginner' is the definition.
(I know that rookie is a type of beginner)
'fleece that is' is the wordplay.
'fleece' becomes 'rook' (I've seen this before).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'for' is the link.
(Other definitions for rookie that I've seen before include "Novice" , "New recruit as to army" , "untrained soldier" , "Callow recruit" , "New (police) recruit (colloq.)" .)