Expansive part in new idea (4)
I believe the answer is:
wide
'expansive' is the definition.
(I've seen this before)
'part in new idea' is the wordplay.
'part in' indicates the answer is hidden within the clue.
'WIDE' can be found hidden inside 'new idea'.
(Other definitions for wide that I've seen before include "Spacious; an extra" , "lax in morals" , "roomy" , "Broad - extra" , "Unplayable ball (cricket)" .)