Rescue vessel returns with knowledge of monster (6)
I believe the answer is:
kraken
'monster' is the definition.
(I know that Kraken is an sea monster)
'rescue vessel returns with knowledge' is the wordplay.
'rescue vessel' becomes 'ark' (rescue ship in the Bible).
'returns' is a reversal indicator.
'with' means one lot of letters go next to another.
'knowledge' becomes 'ken' (ken is a kind of knowledge).
'ark' reversed gives 'kra'.
'kra'+'ken'='KRAKEN'
'of' acts as a link.
(Other definitions for kraken that I've seen before include "Legendary monster of Norwegian seas" , "Sea monster that sleepeth (Tennyson)" , "A legendary sea monster of Norwegian myth" , "Mythical sea-monster off Norway's coast" , "The -- Wakes (John Wyndham)" .)