Carriage is about right (4)
I believe the answer is:
cart
'carriage' is the definition.
(thesaurus)
'about right' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'ca'+'rt'='CART'
'is' acts as a link.
(Other definitions for cart that I've seen before include "Wheelbarrow" , "Hump" , "Carrying vehicle" , "Horse-drawn vehicle" , "Old two-wheeled goods vehicle" .)