Hailstones outside, ate top meal inside; got nasty correspondence (4,4)
I believe the answer is:
hate mail
'correspondence' is the definition.
(I have seen 'Offensive correspondence ' mean 'hate mail' so perhaps 'correspondence' could also mean 'hate mail')
'hailstones outside ate top meal inside got nasty' is the wordplay.
'hailstones' becomes 'hal' (I can't explain this - if you can you should give a lot more credence to this answer).
'outside' is an insertion indicator (some letters go outside others).
'top' says to take the initial letters (in a down clue, the first letter is top).
'got nasty' indicates an anagram (I've seen 'get nasty' mean this (a nasty or bad spelling of the letters)).
The first letters of 'meal inside' is 'mi'.
'ate'+'mi'='atemi'
'atemi' with letters rearranged gives 'temai'.
'hal' enclosing 'temai' is 'HATE MAIL'.
Can you help me to learn more?
(Other definitions for hate mail that I've seen before include "communication with animosity - with 19a" , "Nasty letters" , "Abusive letter that expresses loathing" , "hostile correspondence" , "Poison-pen letters" .)