Boring, it's rejected by engineers - an unspecified amount (8)
I believe the answer is:
tiresome
'boring' is the definition.
(I know that boring can be written as tiresome)
'it's rejected by engineers an unspecified amount' is the wordplay.
'rejected' says the letters should be written in reverse.
'by' says to put letters next to each other.
'engineers' becomes 're' (Royal Engineers).
'an unspecified amount' becomes 'some' (I've seen this in another clue).
'it' backwards is 'ti'.
'ti'+'re'+'some'='TIRESOME'
(Other definitions for tiresome that I've seen before include "Causing boredom" , "Boring, irksome (colloq.)" , "irritating" , "Annoying, being a nuisance" , "Causing impatience" .)