Engineer's instrument is one that's not working (8)
I believe the answer is:
organise
'engineer's' is the definition.
('engineer'->'organize' is in my internal thesaurus and 'ize'->'ise')
'instrument is one that's not working' is the wordplay.
'instrument' becomes 'organ' (organ is a kind of instrument).
'that's not' suggests deleting specific letters ('that has not' as in 'that doesn't have').
'working' becomes 'on' (on can mean working or activated).
'one' with 'on' taken out is 'e'.
'organ'+'is'+'e'='ORGANISE'
(Other definitions for organise that I've seen before include "form" , "Set up, co-ordinate" , "Arrange in a systematic or orderly way" , "Catalogue" , "Arrange and co-ordinate activity" .)