Is up to mythical bird, care of hot wind (7)
I believe the answer is:
sirocco
'hot wind' is the definition.
(thesaurus)
'is up to mythical bird care of' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'to mythical bird' becomes 'roc' (I've seen this before).
'care of' becomes 'co' (c/o can mean 'care of' when addressing mail).
'is' back-to-front is 'si'.
'si'+'roc'+'co'='SIROCCO'
(Other definitions for sirocco that I've seen before include "Hot air" , "Hot wind that blows across the Mediterranean" , "Wind from the desert" , "Dust storm" , "A hot wind blowing from North Africa across the Mediterranean" .)