Deny engineers turned up, ruining a diet (9)
I believe the answer is:
repudiate
'deny' is the definition.
(I know that deny is a more specific form of repudiate)
'engineers turned up ruining a diet' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'turned' says the letters should be written in reverse.
'ruining' is an anagram indicator.
'up' in reverse letter order is 'pu'.
'a'+'diet'='adiet'
'adiet' with letters rearranged gives 'diate'.
're'+'pu'+'diate'='REPUDIATE'
(Other definitions for repudiate that I've seen before include "Reject as untrue or unjust" , "Deny the truth of (something)" , "Make a disclaimer" , "Renounce, disown" , "Refuse to be associated with" .)