Star I'm backing to take off? (5)
I believe the answer is:
minus
'off?' is the definition.
(thesaurus)
'star i'm backing' is the wordplay.
'star' becomes 'sun' (the Sun is a star).
'backing' shows that the letters should be reversed in order.
'sun'+'im'='sunim'
'sunim' back-to-front is 'MINUS'.
'to take' is the link.
(Other definitions for minus that I've seen before include "Deprived of" , "Negative sign - negative thing" , "Lacking; negative" , "Less, without" , "'Take away' sign" .)