Nature reserve dispatching first rescue vessel (3)
I believe the answer is:
ark
'rescue vessel' is the definition.
(rescue ship in the Bible)
'nature reserve dispatching first' is the wordplay.
'nature reserve' becomes 'park'.
'dispatching first' means to remove the first letter.
'park' with its first letter taken off is 'ARK'.
(Other definitions for ark that I've seen before include "Sacred chest" , "Flood vessel; box (arch.)" , "Large vessel" , "Great biblical boat" , "Noah's ship" .)