You and I sort letters in tall buildings (6)
I believe the answer is:
towers
'tall buildings' is the definition.
(I've seen this in another clue)
'you and i sort letters in' is the wordplay.
'you and i' becomes 'we'.
'letters' indicates an anagram.
'in' means one lot of letters goes inside another.
'sort' is an anagram of 'tors'.
'we' put inside 'tors' is 'TOWERS'.
(Other definitions for towers that I've seen before include "Keeps" , "'Fawlty - - ' (TV)" , "Tall thin structures" , "Tall, thin structure" , "Turrets" .)