Old woman has grabbed one’s abnormally large shrub (6)
I believe the answer is:
mimosa
'shrub' is the definition.
(mimosa is a kind of shrub)
'old woman has grabbed one's abnormally large' is the wordplay.
'old woman' becomes 'ma' (both can mean someone's mother).
'has grabbed' is an insertion indicator (inserted letters are grabbed inside).
'one's' becomes 'I'm' ('one is' is a formal way of saying 'I am').
'abnormally large' becomes 'OS' (abbreviation for outsized).
'im'+'os'='imos'
'ma' going around 'imos' is 'MIMOSA'.
(Other definitions for mimosa that I've seen before include "Sensitive thing growing" , "Yellow-flowered acacia tree" , "Shrub - I'm Amos (anag)" , "Tropical plant" , "So maim with tropical shrub bearing fragrant flowers" .)