Begin with famous sheep (5,2)
I believe the answer is:
start up
'begin' is the definition.
('start up' can be a synonym of 'begin')
'with famous sheep' is the wordplay.
'with' means one lot of letters go next to another.
'famous' becomes 'star' (I've seen this before).
'sheep' becomes 'tup' (tup is a kind of sheep).
'star' put next to 'tup' is 'START-UP'.
(Other definitions for start up that I've seen before include "Begin" , "Action of initiating" , "fresh concern" , "Found" , "Newly-established firm" .)