Back again with that heartless air (4)
I believe the answer is:
rear
'back' is the definition.
(synonyms)
'again with that heartless air' is the wordplay.
'again' becomes 're'.
'with' says to put letters next to each other.
'that heartless' says to hollow out the word (remove centre letters).
'air' with its middle taken out is 'ar'.
're'+'ar'='REAR'
(Other definitions for rear that I've seen before include "Stern" , "Buttocks - rise up" , "Bring up or stand up on hind legs" , "Bring up - back end" , "Posterior" .)