It's unusual to return right on time (4)
I believe the answer is:
rare
'it's unusual' is the definition.
(thesaurus)
'return right on time' is the wordplay.
'return' says the letters should be written backwards.
'right' becomes 'r' (common abbreviation).
'on' is a charade indicator (letters next to each other).
'time' becomes 'era' (I've seen this before**).
'r' after 'era' is 'erar'.
'erar' back-to-front is 'RARE'.
'to' is the link.
(Other definitions for rare that I've seen before include "Might a four-leaf clover be so" , "thin on the ground" , "Bloody (of steaks)" , "may be bloody" , "Rear up, being scarce" .)