Throw up during field day (5)
I believe the answer is:
addle
'throw' is the definition.
(thesaurus)
'up during field day' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'during' says the answer is hidden in the clue.
'eldda' is hidden in the letters of 'field day'.
'eldda' back-to-front is 'ADDLE'.
(Other definitions for addle that I've seen before include "Make confused - go bad" , "Go rotten" , "(Of eggs) become rotten" , "Get rotten" , "becomes muddled" .)