Become confused backing up some field data (5)
I believe the answer is:
addle
'become confused' is the definition.
'backing up some field data' is the wordplay.
'backing up' says the letters should be written in reverse.
'some' indicates a hidden word (some of the parts of the word are used).
'eldda' can be found hidden inside 'field data'.
'eldda' in reverse letter order is 'ADDLE'.
(Other definitions for addle that I've seen before include "Get rotten" , "Make rotten" , "becomes muddled" , "(Of eggs) become rotten" , "Confuse; become rotten" .)