Emphasise that the accountant returned the money (6)
I believe the answer is:
accent
'emphasise' is the definition.
('accent' can be a synonym of 'emphasize')
'the accountant returned the money' is the wordplay.
'the accountant' becomes 'ca' (abbreviation for chartered accountant).
'returned' says the letters should be written backwards.
'the money' becomes 'cent' (I've seen this before).
'ca' written backwards gives 'ac'.
'ac'+'cent'='ACCENT'
'that' is the link.
(Other definitions for accent that I've seen before include "Mark over a vowel, perhaps" , "Pronunciation in a locality" , "Grave, perhaps" , "Underline" , "Stress - manner of speech" .)