English officer opening US lawman’s letter (5)
I believe the answer is:
delta
'letter' is the definition.
(NATO phonetic alphabet)
'english officer opening us lawman's' is the wordplay.
'english' becomes 'e' (abbreviation).
'officer' becomes 'lt' (I've seen this in other clues).
'opening' is an insertion indicator (some letters open a gap in others).
'us lawman' becomes 'da' (district attorney).
'e'+'lt'='elt'
'elt' inserted into 'da' is 'DELTA'.
(Other definitions for delta that I've seen before include "Wide silted river mouth" , "The Greek D (5)" , "Area where a river divides before entering the sea" , "sort of wing" , "Alluvial river mouth" .)