Letter to everybody above average height (4)
I believe the answer is:
tall
'above average height' is the definition.
(I've seen this before)
'letter to everybody' is the wordplay.
'letter' becomes 't' ().
'to everybody' becomes 'all' (all people).
't'+'all'='TALL'
(Other definitions for tall that I've seen before include "Vertically large" , "Higher than the average person" , "hard to believe in" , "Above average height" , "Of great height" .)