Dinghy for one dropping off old fly-by-night? (3)
I believe the answer is:
bat
'fly-by-night?' is the definition.
(animal which flies at night)
'dinghy for one dropping off old' is the wordplay.
'dinghy for one' becomes 'boat' (dinghy is a kind of boat).
'dropping off' suggests deleting specific letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'boat' with 'o' removed is 'BAT'.
(Other definitions for bat that I've seen before include "A mammal; a club" , "Club used in cricket" , "Cricketer; mammal" , "Nocturnal animal - racket" , "Small mammal or cricket striker" .)