Biographer, dandy following old boy around (7)
I believe the answer is:
boswell
'biographer' is the definition.
(I've seen this before)
'dandy following old boy around' is the wordplay.
'dandy' becomes 'swell' (both can informally mean excellent or satisfactory).
'following' is a charade indicator (letters next to each other).
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'around' says the letters should be written in reverse.
'ob' backwards is 'bo'.
'swell' after 'bo' is 'BOSWELL'.
(Other definitions for boswell that I've seen before include "Writer of Johnson's life" , "Companion and biographer of Samuel Johnson" , "diarist" , "Samuel Johnson's biographer" , "James --, biographer" .)