Engineers turn up impressive formula (6)
I believe the answer is:
recipe
'formula' is the definition.
('recipe' can be a synonym of 'formula')
'engineers turn up impressive' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'turn up' says the letters should be written in reverse.
'impressive' becomes 'epic' (I've seen this before).
'epic' reversed gives 'cipe'.
're'+'cipe'='RECIPE'
(Other definitions for recipe that I've seen before include "Directions for making something" , "Kitchen instructions" , "List of ingredients" , "Instruction for cooking" , "Cooking instructions" .)