Bound reportedly to drift (4)
I believe the answer is:
tide
'to drift' is the definition.
(I know that tide is a kind of be adrift)
'bound reportedly' is the wordplay.
'bound' becomes 'tied' ('tie' can be a synonym of 'bind').
'reportedly' indicates a 'sounds like' (homophone) clue.
'tied' sounds like 'TIDE'.
(Other definitions for tide that I've seen before include "Ocean phase" , "Periodic rise and fall of the sea" , "Something going in and out" , "It ebbs and flows" , "Current, ebb" .)