Heard of line that was supposedly 300 cubits long (3)
I believe the answer is:
ark
'was supposedly 300 cubits long' is the definition.
(ref to supposed length of Noah's Ark)
'heard of line' is the wordplay.
'heard of' indicates a 'sounds like' (homophone) clue.
'line' becomes 'arc' (arc is a kind of line).
'arc' sounds like 'ARK'.
'that' is the link.
(Other definitions for ark that I've seen before include "old vessel" , "Boat built to withstand great flood" , "The earliest vessel?" , "Boat built to withstand floods" , "Rescue ship" .)