Person singing in new boat (4)
I believe the answer is:
nark
'person singing' is the definition.
(sing can informally mean to talk to the police)
'new boat' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'boat' becomes 'ark' (ark is a kind of boat).
'n'+'ark'='NARK'
'in' is the link.
(Other definitions for nark that I've seen before include "Annoy; informer (sl.)" , "Nettle - informer" , "Bug" , "Informer - irritate" , "Cooper's .... was old police informer" .)