Extra bus shelters working (5)
I believe the answer is:
bonus
'extra' is the definition.
(thesaurus)
'bus shelters working' is the wordplay.
'shelters' means one lot of letters goes inside another.
'working' becomes 'on'.
'bus' placed around 'on' is 'BONUS'.
(Other definitions for bonus that I've seen before include "Payment additional to the expected or usual" , "Extra payment, may be performance-related" , "Extra payment for good results" , "Premium" , "Money on top of wages or salary" .)