Impress when high as an engineer (8)
I believe the answer is:
shanghai
'impress' is the definition.
('shanghai' can be a synonym of 'impress')
'high as an engineer' is the wordplay.
'engineer' is an anagram indicator.
'high'+'as'+'an'='highasan'
'highasan' with letters rearranged gives 'SHANGHAI'.
'when' is the link.
(Other definitions for shanghai that I've seen before include "Kidnap - Chinese port" , "Take to sea" , "How to impress a sailor" , "Port city on the Yangtze estuary" , "Press into service" .)