Bush -- 'I am very big in Massachusetts' (6)
I believe the answer is:
mimosa
'bush' is the definition.
(mimosa is a kind of bush)
'i am very big in massachusetts' is the wordplay.
'i am' becomes 'im'.
'very big' becomes 'os' (abbreviation for outsize).
'in' means one lot of letters goes inside another.
'massachusetts' becomes 'ma'.
'im'+'os'='imos'
'imos' going within 'ma' is 'MIMOSA'.
(Other definitions for mimosa that I've seen before include "Type of acacia tree" , "Tropical tree with yellow flowers" , "Tree with scented clustered flowers" , "Sensitive plant genus; so maim (anag.)" , "Australian 15 across" .)