Abandoned decent gel formula (9)
I believe the answer is:
neglected
'abandoned' is the definition.
(thesaurus)
'decent gel formula' is the wordplay.
'formula' indicates anagramming the letters (the letters are the ingredients or formula).
'decent'+'gel'='decentgel'
'decentgel' with letters rearranged gives 'NEGLECTED'.
(Other definitions for neglected that I've seen before include "Ignored or failed to do" , "Overgrown" , "Ignored, left undone" , "Overlooked" , "Not cared for" .)