Peter has developed zero applications that are very secure (4,2,6)
I believe the answer is:
safe as houses
'are very secure' is the definition.
(I've seen this before)
'peter has developed zero applications' is the wordplay.
'peter' becomes 'safe' (slang term).
'developed' is an anagram indicator.
'zero' becomes 'O' (O resembles 0).
'applications' becomes 'uses' (I've seen this before).
'has' with letters rearranged gives 'ash'.
'safe'+'ash'+'o'+'uses'='SAFE AS HOUSES'
'that' is the link.
(Other definitions for safe as houses that I've seen before include "Thoroughly secure" , "Not in the least dangerous" , "Not in the least at risk" .)