Singer's last to join rescue vessel (4)
I believe the answer is:
nark
'singer's' is the definition.
(both can mean a police informant)
'last to join rescue vessel' is the wordplay.
'last to' suggests the final letters.
'rescue vessel' becomes 'ark' (rescue ship in the Bible).
The last letter of 'join' is 'n'.
'n'+'ark'='NARK'
(Other definitions for nark that I've seen before include "Stool pigeon" , "Rat" , "Nettle - informer" , "a nuisance down under" , "Annoy; informer (sl.)" .)