Distrust sappers abandoning temporary fortification (5)
I believe the answer is:
doubt
'distrust' is the definition.
(synonyms)
'sappers abandoning temporary fortification' is the wordplay.
'sappers' becomes 'RE' (nickname for the Royal Engineers).
'abandoning' is a deletion indicator.
'temporary fortification' becomes 'redoubt' (I've seen this before).
'redoubt' with 're' taken away is 'DOUBT'.
(Other definitions for doubt that I've seen before include "One certainly can't have it" , "Uncertainty, unsureness" , "Distrust" , "Have reservations or be uncertain about" , "Hesitate to believe" .)