A substitute about to do the job (7)
I believe the answer is:
reserve
'a substitute' is the definition.
(synonyms)
'about to do the job' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to do the job' becomes 'serve' (I've seen this in another clue).
're'+'serve'='RESERVE'
(Other definitions for reserve that I've seen before include "Reticence" , "not in the front line" , "Wine kept" , "cool" , "Order" .)