Swell in endless bliss (5)
I believe the answer is:
heave
'swell' is the definition.
(I've seen this in another clue)
'endless bliss' is the wordplay.
'endless' means to remove the last letter.
'bliss' becomes 'heaven' (I've seen this before).
'heaven' with its last letter taken away is 'HEAVE'.
'in' is the link.
(Other definitions for heave that I've seen before include "Throw, hurl" , "Hoist" , "Produce (a sigh); retch" , "Pitch" , "Lift or haul with great effort" .)