Giant also upset shows sentimental feeling (9)
I believe the answer is:
nostalgia
'sentimental feeling' is the definition.
(I've seen this before)
'giant also upset' is the wordplay.
'upset' is an anagram indicator.
'giant'+'also'='giantalso'
'giantalso' with letters rearranged gives 'NOSTALGIA'.
'shows' acts as a link.
(Other definitions for nostalgia that I've seen before include "fondly looking back" , "Longing for things past" , "Backward-looking sentiment" , "Love for the past" , "A sentimental longing of analogist" .)