Peter's so reliable! (4)
I believe the answer is:
safe
This clue is a double definition.
'peter's' is the first definition.
(slang term)
'reliable' is the second definition.
(I've seen this before)
'so' acts as a link.
(Other definitions for safe that I've seen before include "Metal case for storing valuables" , "Locked store for money etc" , "Metal box for security" , "Strong metal box to store valuables" , "Uninjured, secure" .)