Detective, hard on upset mum in London borough (8)

'london borough' is the definition.
(I know that Lewisham is a London borough)

'detective hard on upset mum' is the wordplay.
'detective' becomes 'lewis' (Lewis is Inspector Morse's sidekick).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'upset' says the letters should be written backwards.
'mum' becomes 'ma' (both can mean mother).
'ma' written backwards gives 'am'.
'lewis'+'h'+'am'='LEWISHAM'

'in' is the link.