Giant also upset in homesickness (9)
I believe the answer is:
nostalgia
'homesickness' is the definition.
(I've seen this in another clue)
'giant also upset' is the wordplay.
'upset' indicates an anagram.
'giant'+'also'='giantalso'
'giantalso' is an anagram of 'NOSTALGIA'.
'in' is the link.
(Other definitions for nostalgia that I've seen before include "Longing for what's gone" , "Fond remembrance" , "A longing for past times" , "fondly looking back" , "Attraction to the past" .)