Engineers engaged in major engineering project showing vision (5)
I believe the answer is:
'vision' is the definition.
(dream is a kind of vision)
'engineers engaged in major engineering project' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'engaged in' indicates putting letters inside.
'major engineering project' becomes 'dam' (a dam is a type of engineering work).
're' going inside 'dam' is 'DREAM'.
'showing' is the link.
(Other definitions for dream that I've seen before include "Sleeper's vision" , "King had one" , "Cherished hope or ideal" , "Sleeping thoughts" , "Switch off" .)