Gloom as some captives send a signal back (7)
I believe the answer is:
sadness
'gloom' is the definition.
(thesaurus)
'some captives send a signal back' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'back' says the letters should be written in reverse.
'ssendas' is hidden in the letters of 'captives send a signal'.
'ssendas' back-to-front is 'SADNESS'.
'as' acts as a link.
(Other definitions for sadness that I've seen before include "Emotion experienced when unhappy" , "regret" , "Sorrow, unhappiness" , "no joy there" , "Despondency" .)