Giant blunder in field by Royal Engineers (4)
I believe the answer is:
ogre
'giant' is the definition.
(ogre is a kind of giant)
'blunder in field by royal engineers' is the wordplay.
'blunder in field' becomes 'og' (I can't explain this - if you can you should believe this answer much more).
'by' says to put letters next to each other.
'royal engineers' becomes 're'.
'og'+'re'='OGRE'
Can you help me to learn more?
(Other definitions for ogre that I've seen before include "Giant of folklore, eats humans" , "Wicked giant" , "Jack's giant perhaps" , "character in 'Puss in Boots'" , "bad-tempered person" .)