Growing rat eating very much (8)
I believe the answer is:
swelling
'growing' is the definition.
(swelling is a kind of growing)
'rat eating very much' is the wordplay.
'rat' becomes 'sing' (both can mean to inform on someone).
'eating' means one lot of letters goes inside another.
'very much' becomes 'well' (well can mean very much or to a great extent).
'sing' going around 'well' is 'SWELLING'.
(Other definitions for swelling that I've seen before include "Tumour" , "getting bigger" , "Bulge" , "Abnormal enlargement" , "Nodule" .)