Wistfulness may be shown by lots again in resort (9)
I believe the answer is:
nostalgia
'wistfulness' is the definition.
(I've seen this before)
'be shown by lots again in resort' is the wordplay.
'be shown by' says to put letters next to each other.
'in resort' indicates an anagram (the letters are re-sorted).
'lots'+'again' is 'lotsagain'.
'lotsagain' is an anagram of 'NOSTALGIA'.
'may' acts as a link.
(Other definitions for nostalgia that I've seen before include "Sentimental longing for the past" , "Sentimental feeling of analogist" , "Attraction to the past" , "Sentimental feeling for old times" , "That may bring on homesickness" .)