Unlikely book being rejected is welcomed by engineers (6)
I believe the answer is:
remote
'unlikely' is the definition.
(I've seen this before)
'book being rejected is welcomed by engineers' is the wordplay.
'book' becomes 'tome' (tome is a kind of book).
'being rejected' shows that the letters should be reversed in order.
'is welcomed by' indicates putting letters inside (I've seen this in other clues).
'engineers' becomes 're' (Royal Engineers).
'tome' backwards is 'emot'.
'emot' going within 're' is 'REMOTE'.
(Other definitions for remote that I've seen before include "Stand-offish" , "Far off; outside (chance)" , "Controller of programmes" , "Very far away like meteor" , "Isolated" .)