Draw large aquatic mammal close to quay (7)
I believe the answer is:
lottery
'draw' is the definition.
(I've seen this before)
'large aquatic mammal close to quay' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'aquatic mammal' becomes 'otter' (otters live by water).
'close to' says to take the final letters (the close/ending of).
The final letter of 'quay' is 'y'.
'l'+'otter'+'y'='LOTTERY'
(Other definitions for lottery that I've seen before include "chancy business" , "Raffle" , "Prize draw" , "Random prize game" , "Matter of chance" .)