Homesickness caused by reacting to a signal (9)
I believe the answer is:
nostalgia
'homesickness' is the definition.
(thesaurus)
'reacting to a signal' is the wordplay.
'reacting' indicates an anagram.
'to'+'a'+'signal'='toasignal'
'toasignal' with letters rearranged gives 'NOSTALGIA'.
'caused by' acts as a link.
(Other definitions for nostalgia that I've seen before include "Backward-looking sentiment" , "Fond memories" , "Longing for things past" , "Looking back with fondness on the past" , "Longing for something past - isn't a goal (anag)" .)