Fancy reason to save instant messages (6)
I believe the answer is:
whimsy
'fancy' is the definition.
(thesaurus)
'reason to save instant messages' is the wordplay.
'reason' becomes 'why' (why is a kind of reason).
'to save' indicates putting letters inside.
'instant messages' becomes 'IMs' (abbreviation).
'why' placed around 'ims' is 'WHIMSY'.
(Other definitions for whimsy that I've seen before include "Odd or fanciful idea" , "Quaint fanciful humour" , "Fanciful ideas" , "Gentle fantasy" , "Capricious notion" .)