A diary, any sort, one saves up for (1,5,3)
I believe the answer is:
a rainy day
'one saves up for' is the definition.
(as in saving for a rainy day)
'a diary any sort' is the wordplay.
'sort' is an anagram indicator.
'diary'+'any'='diaryany'
'diaryany' with letters rearranged gives 'rainyday'.
'a'+'rainyday'='A RAINY DAY'
(Other definitions for a rainy day that I've seen before include "preferably not July 15th" , "You should save money for this wet future" , "Mushroom could prove useful in" , "Possible future time of need" , "Something to plan for" .)