Partner with swine fever loses nerves (4)
I believe the answer is:
wife
'partner' is the definition.
(wife is a kind of partner)
'swine fever loses nerves' is the wordplay.
'loses' is a deletion indicator.
'swine'+'fever'='swinefever'
'swinefever' with 'nerves' removed is 'WIFE'.
'with' is the link.
(Other definitions for wife that I've seen before include "Better half" , "Husband's partner" , "Spouse" , "Dutch (slang)" , "Hubby's other half" .)