They persistently criticise new vessels (5)
I believe the answer is:
narks
'they persistently criticise' is the definition.
(nark can mean someone who regularly criticises)
'new vessels' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'vessels' becomes 'arks' (ark is a kind of vessel).
'n'+'arks'='NARKS'
(Other definitions for narks that I've seen before include "Annoys; informers (sl.)" , "Irritates" , "Coppers' ..... are slang police informers" , "singers" .)