Informer's name on chest (4)
I believe the answer is:
nark
'informer's' is the definition.
(nark is a kind of informer)
'name on chest' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'on' says to put letters next to each other.
'chest' becomes 'ark' (ark is a kind of chest).
'n'+'ark'='NARK'
(Other definitions for nark that I've seen before include "upset" , "Bug" , "Cause minor annoyance" , "Habitual grumbler" , "Nettle - informer" .)