Engage in new project? Not half! (5)
I believe the answer is:
enter
'engage in' is the definition.
(thesaurus)
'new project? not half' is the wordplay.
'new project?' becomes 'enterprise' (I've seen this before).
'not half' suggests halving the letters.
'enterprise' cut in half is 'ENTER'.
(Other definitions for enter that I've seen before include "Introduce" , "Treen (anag.)" , "Record - stage direction - come in" , "Get in" , "Make a record of" .)