Soldiers admit post brought about depression (7)
I believe the answer is:
sadness
'depression' is the definition.
(I've seen this before)
'soldiers admit post brought about' is the wordplay.
'soldiers' becomes 'sas' (Special Air Service).
'admit' is an insertion indicator.
'post' becomes 'send' (synonyms).
'brought about' says the letters should be written backwards.
'sas' placed around 'send' is 'ssendas'.
'ssendas' in reverse letter order is 'SADNESS'.
(Other definitions for sadness that I've seen before include "Sorrow, unhappiness" , "Gloom" , "distress" , "blues" , "Sorrowful quality" .)