Empty sort of pun about a preposition (4)
I believe the answer is:
upon
'a preposition' is the definition.
(I've seen this before)
'empty sort of pun about' is the wordplay.
'empty' becomes 'o'.
'sort of' is an anagram indicator.
'about' means one lot of letters goes inside another.
'pun' with letters rearranged gives 'upn'.
'o' going into 'upn' is 'UPON'.
(Other definitions for upon that I've seen before include "Preposition" , "'''It came ... the midnight clear'' (carol) (4)'" , "voucher, business stopped" , "Straddling" , "(Placed) on top of" .)