Peter Rabbit’s head is more secure (5)
I believe the answer is:
'more secure' is the definition.
('safe' can be a synonym of 'secure')
'peter rabbit's head' is the wordplay.
'peter' becomes 'safe' (slang term).
'head' suggests taking the first letters.
The first letter of 'rabbits' is 'r'.
'is' acts as a link.
(Other definitions for safer that I've seen before include "Less endangered" , "Not so hairy" , "Fears (anag)" , "Less adventurous" , "Less hazardous" .)