Grim, the old lady about to grab taxi (7)
I believe the answer is:
macabre
'grim' is the definition.
(thesaurus)
'the old lady about to grab taxi' is the wordplay.
'the old lady' becomes 'ma'.
'about' becomes 're' (regarding**).
'to grab' means one lot of letters goes inside another.
'taxi' becomes 'cab' ('cab' can be a synonym of 'taxi').
'ma'+'re'='mare'
'mare' placed around 'cab' is 'MACABRE'.
(Other definitions for macabre that I've seen before include "Gruesome and weird" , "Morbidly repellent" , "Dark and disturbing" , "Ghostly appearance" , "Gruesome, morbid" .)