Police informer’s name found on old ship (4)
I believe the answer is:
nark
'police informer's' is the definition.
(I've seen this in another clue)
'name found on old ship' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'found on' says to put letters next to each other.
'old ship' becomes 'ark' (as in Noah's ark).
'n'+'ark'='NARK'
(Other definitions for nark that I've seen before include "Keep an eye" , "Police spy" , "Annoy (colloq.); informer" , "Rat" , "Annoy - informant" .)