Delight, as it were, as rescue vessel breaks through earth (6)
I believe the answer is:
darken
'delight as it were' is the definition.
(as in de-light)
'rescue vessel breaks through earth' is the wordplay.
'rescue vessel' becomes 'ark' (I've seen this before).
'breaks through' means one lot of letters goes inside another.
'earth' becomes 'den'.
'ark' placed within 'den' is 'DARKEN'.
'as' acts as a link.
(Other definitions for darken that I've seen before include "Cloud over" , "shade" , "Make less light" , "Cast a shadow over" , "Grow less light" .)