Build up layer inserting cube (8)
I believe the answer is:
heighten
'build up' is the definition.
(thesaurus)
'layer inserting cube' is the wordplay.
'layer' becomes 'hen' (I've seen this before).
'inserting' indicates putting letters inside.
'cube' becomes 'eight' (8 is a cube number).
'hen' going around 'eight' is 'HEIGHTEN'.
(Other definitions for heighten that I've seen before include "Make more intense, say awareness" , "effect some improvement" , "Increase in amount of degree" , "Elevate - intensify" , "Intensify, enhance" .)