Favourite left out? That's brave (6)
I believe the answer is:
daring
'brave' is the definition.
(thesaurus)
'favourite left out?' is the wordplay.
'favourite' becomes 'darling' ('darling' can be a synonym of 'favourite').
'left' becomes 'l' (common abbreviation).
'out?' indicates named letters should be taken away.
'darling' with 'l' taken out is 'DARING'.
'that's' is the link.
(Other definitions for daring that I've seen before include "Bold, willing to take chance" , "'Adventurous, brave (6)'" , "Adventurous or audaciously bold" , "Intrepid" , "Boldness" .)