In defending Leningrad from narrow valley (4)
I believe the answer is:
glen
'narrow valley' is the definition.
(I've seen this in another clue)
'in defending leningrad from' is the wordplay.
'in' indicates a hidden word.
'GLEN' is hidden in the letters of 'defending leningrad from'.
(Other definitions for glen that I've seen before include "Narrow secluded valley in mountains" , "Depression in Scotland" , "Valley, dale" , "Narrow, deep mountain valley" , "Narrow valley with a stream" .)