Boring - it's rejected by engineers - an unspecified number (8)
I believe the answer is:
tiresome
'boring' is the definition.
(synonyms)
'it's rejected by engineers an unspecified number' is the wordplay.
'rejected' says the letters should be written in reverse.
'by' is a charade indicator (letters next to each other).
'engineers' becomes 're' (Royal Engineers).
'an unspecified number' becomes 'some' (I've seen this before).
'it' reversed gives 'ti'.
'ti'+'re'+'some'='TIRESOME'
(Other definitions for tiresome that I've seen before include "Wearying" , "irritating" , "Causing impatience" , "Tedious and boring" , "So metier may be boring (8)" .)