Motorboat is trapped in Corfu weir every so often (7)
I believe the answer is:
cruiser
'motorboat' is the definition.
(cruiser is a kind of motorboat)
'is trapped in corfu weir every so often' is the wordplay.
'trapped in' means one lot of letters goes inside another.
'every so often' indicates alternate letters.
'corfu'+'weir'='corfuweir'
The alternate letters of 'corfuweir' are 'cruer'.
'is' going within 'cruer' is 'CRUISER'.
(Other definitions for cruiser that I've seen before include "Leisure yacht; warship" , "Ship" , "Speedy warship" , "Large warship" , "Pleasure craft" .)