One time, yours truly sent back individual unit (4)
I believe the answer is:
item
'individual unit' is the definition.
(I've seen this before)
'one time yours truly sent back' is the wordplay.
'one' becomes 'i' (Roman numeral).
'time' becomes 't' (abbreviation).
'yours truly' becomes 'me' (the person who set this crossword).
'sent back' says the letters should be written in reverse.
'me' in reverse letter order is 'em'.
'i'+'t'+'em'='ITEM'
(Other definitions for item that I've seen before include "Object, detail" , "A thing or unit" , "'One thing, poor mite (4)'" , "couple linked" , "One thing - two people, perhaps" .)