Relish keeping twin-hulled vessel insured at first, in case (8)

'case' is the definition.
(locative is a kind of case)

'relish keeping twin-hulled vessel insured at first' is the wordplay.
'relish' becomes 'love'.
'keeping' is an insertion indicator.
'twin-hulled vessel' becomes 'cat' (short for catamaran, type of ship).
'at first' indicates taking the first letters.
The first letter of 'insured' is 'i'.
'cat'+'i'='cati'
'love' placed around 'cati' is 'LOCATIVE'.

'in' is the link.