Engineers beat off discount (6)
I believe the answer is:
rebate
'discount' is the definition.
(I know that discount can be written as rebate)
'engineers beat off' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'off' indicates an anagram.
'beat' is an anagram of 'bate'.
're'+'bate'='REBATE'
(Other definitions for rebate that I've seen before include "Partial refund, reduction in payment" , "Cash refund" , "Give back (money)" , "Money back, partial refund" , "price reduction" .)