Conservative fails to win contracts (6)
I believe the answer is:
closes
'contracts' is the definition.
(to contract can mean to close)
'conservative fails to win' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'fails to win' becomes 'loses' ('lose' can be a synonym of 'fail to win').
'c'+'loses'='CLOSES'
(Other definitions for closes that I've seen before include "Wind up" , "Shuts" , "comes to an end" .)