Contract ultimately broken? One's fired! (6)
I believe the answer is:
narrow
'contract' is the definition.
(I know that contract can be written as narrow)
'ultimately broken? one's fired' is the wordplay.
'ultimately' suggests the final letters.
'one's fired' becomes 'arrow' (an archer fires arrows).
The final letter of 'broken' is 'n'.
'n'+'arrow'='NARROW'
(Other definitions for narrow that I've seen before include "Short of breadth" , "Cramped" , "Vessel designed for canals" , "Pinched" , "Lacking tolerance" .)