Make known Imogen is to leave (6)
I believe the answer is:
impart
'make known' is the definition.
(synonyms)
'imogen is to leave' is the wordplay.
'imogen is' becomes 'I'm' (where the crossword setter is 'Imogen').
'to leave' becomes 'part' (parting means leaving or separating).
'im'+'part'='IMPART'
(Other definitions for impart that I've seen before include "Reveal about armpit" , "Hand on" , "17 down (anag)" , "Give, say information, make known" , "Communicate, pass on" .)