Enlargement, expansion (6)
I believe the answer is:
growth
I believe this clue is a double definition.
'enlargement' is the first definition.
(thesaurus)
'expansion' is the second definition.
(I've seen this in another clue)
(Other definitions for growth that I've seen before include "Augmentation" , "Process of development" , "Process of increase" , "Tumour - enlargement" , "Increase - development - tumour" .)