More than one basin submerges - (5)
SINKS: double def
vassals take wrong end of ash for ships. (7)
VESSELS (æ switch: solved by Richard)
Wit and aspiration, alongside, (4)
WITH (aspirated "wit": thanks, M!)
hold up to take supper at harbour - (7)
SUP + PORT
pint (swallowed whole?) gives purpose. (5)
P[O]INT
Foremost three first (or four, we hear) (3)
FOR(-emost), FOUR homophone (no definition????)
recorded observations a tad misplaced. (4)
DATA* (thanks, Richard)
A success without us backing admission, (6)
A[-su]CCESS (thanks, Richard (again))
with race of batons racing depositor. (5)
LAYER* (or RELAY*) ("racing" indicates anagram - thanks, M!)
Put together, we get
Sinks vessels with support point for data access layer (4,4,3)
Sinks vessels
POTS sinks(v) in billiards,vessels in cooking
(with) support point
SPOT (double def: to "spot" in the military or in water sports is to watch out while someone else is occupied)
for data access layer