Fly from trenches in retreat (5)
I believe the answer is:
speed
'fly' is the definition.
(thesaurus)
'from trenches in retreat' is the wordplay.
I cannot quite see how this works, but
'from' could be 'e' (I've seen this in other clues) and 'e' is found in the answer.
The remaining letters 'sped' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for speed that I've seen before include "Swiftness" , "Move quickly (and illegally?)" , "Rapidity of movement" , "Rate of progress" , "Film sensitivity to light" .)