Set engineers bad day (5)
I believe the answer is:
ready
'set' is the definition.
(I know that set can be written as ready)
'engineers bad day' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'bad' indicates an anagram.
'day' anagrammed gives 'ady'.
're'+'ady'='READY'
(Other definitions for ready that I've seen before include "Prepare < dough" , "Willing (to do something), prepare" , "Poised for action" , "Intelligent" , "All set to go" .)