Melancholy of elite troops holding post up (7)
I believe the answer is:
sadness
'melancholy' is the definition.
(I've seen this in another clue)
'elite troops holding post up' is the wordplay.
'elite troops' becomes 'SAS' (Special Air Service).
'holding' means one lot of letters goes inside another.
'post' becomes 'send' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'sas' placed around 'send' is 'ssendas'.
'ssendas' reversed gives 'SADNESS'.
'of' acts as a link.
(Other definitions for sadness that I've seen before include "blues" , "Sorrowfulness" , "no joy there" , "Despondency" , "Misery" .)