Canal boat beginning to drift in lagoon aimlessly (7)
I believe the answer is:
gondola
'canal boat' is the definition.
'beginning to drift in lagoon aimlessly' is the wordplay.
'beginning to' indicates taking the first letters.
'in' is an insertion indicator.
'aimlessly' is an anagram indicator (the letters rearranged chaotically or aimlessly).
The initial letter of 'drift' is 'd'.
'lagoon' with letters rearranged gives 'gonola'.
'd' put into 'gonola' is 'GONDOLA'.
(Other definitions for gondola that I've seen before include "Goon lad (anag.)" , "Venice craft" , "Compartment suspended from an airship" , "Shop unit" , "Boat on Venetian canals" .)