In vehicle, husband once left half-cut: has he gone to Monaco? (3,5)
I believe the answer is:
tax exile
'has he gone to monaco?' is the definition.
(a rich person might live in Monaco to pay less tax)
'in vehicle husband once left half-cut' is the wordplay.
'in' is an insertion indicator.
'vehicle' becomes 'taxi' (taxi is a kind of vehicle).
'husband once' becomes 'ex' (an ex-husband).
'half cut' suggests halving the letters.
'taxi' going around 'ex' is 'taxexi'.
'left' halved is 'le'.
'taxexi'+'le'='TAX EXILE'
(Other definitions for tax exile that I've seen before include "Economic migrant" , "rich liver away?" .)