Excessively and completely overwhelming several (9)
I believe the answer is:
fulsomely
'excessively' is the definition.
(fulsomely can mean to an excessive degree)
'completely overwhelming several' is the wordplay.
'completely' becomes 'fully' (in full).
'overwhelming' indicates putting letters inside.
'several' becomes 'some' (similar in meaning).
'fully' placed around 'some' is 'FULSOMELY'.
'and' is the link.
(Other definitions for fulsomely that I've seen before include "displaying sickening obsequiousness" , "In an unctuous, insincere manner" .)